Preface: Assume I am not an ordained Rabbi.
Suppose I'm having lunch with my friend and he asks me "do I have to say a bracha on this dessert?" or "when's the last time I could daven mincha today?" or "am I allowed to carry this back home?" (on Shabbos).
Whatever the case may be, am I allowed to answer him? Or should I just grin and say "CYLOR"?
Highly related: Why is it necessary to ask a Rabbi?
Its only considered a psak if it's a new case that requires original analysis. If you are just reciting a halachik fact, it is not a psak halacha. It does not require a posek and the person can still go and ask someone else. (I heard a similar statement b'shem R' Hershel Schachter).
These Halachot are well documented.
Let's review some of the pertinent Halachot:
ד אָסוּר לְאָדָם לְהוֹרוֹת לִפְנֵי רַבּוֹ לְעוֹלָם, וְכָל הַמּוֹרֶה לְפָנָיו חַיָּב מִיתָה.
הגה: וַאֲפִלּוּ נְטִילַת רְשׁוּת לֹא מְהָנֵי תּוֹךְ שְׁלֹשָׁה פַּרְסָאוֹת, אִם הוּא רַבּוֹ מֻבְהָק. (בֵּית יוֹסֵף בְּשֵׁם תּוֹסָפוֹת פ''ק דְּסַנְהֶדְרִין וּמַהֲרִי''ק שֹׁרֶשׁ ק''ע וַאֲגֻדָּה שָׁם וּסְמַ''ק סוֹף עֲשִׂין י''ג בְּשֵׁם ר''י וּפֶרֶק אַף עַל פִּי) .
One may not decide Halachic questions in one's Rav's presence, or within ~12 KM of one's Rebbe-Muvhak. (A definition of which should probably be discussed in another question, but it is typically defined as one who taught one most of one's Torah knowledge - though see further down.)
וְאִם הוּא רָחוֹק מֵרַבּוֹ י''ב מִיל, וְשָׁאַל לוֹ אָדָם דְּבַר הֲלָכָה בְּדֶרֶךְ מִקְרֶה, יָכוֹל לְהָשִׁיב. אֲבָל לִקְבֹּעַ עַצְמוֹ לְהוֹרָאָה וְלֵישֵׁב וּלְהוֹרוֹת, אֲפִלּוּ הוּא בְּסוֹף הָעוֹלָם, אָסוּר לְהוֹרוֹת עַד שֶׁיָּמוּת רַבּוֹ אוֹ עַד שֶׁיִּתֵּן לוֹ רְשׁוּת.
If he's ~12 KM away from his Rav and someone happens to ask a Halachic question, one may answer. Becoming a Posek - on a permanent basis - even on the other side of the world - is forbidden until his Rav dies or gives him permission.
הגה: וְכָל זֶה בְּרַבּוֹ מֻבְהָק, אֲבָל בְּתַלְמִיד חָבֵר, אֲפִלּוּ תּוֹךְ שְׁלֹשָׁה פַּרְסָאוֹת שָׁרֵי (רי''ף וְרַמְבַּ''ם כִּדְאִיתָא בְּבֵית יוֹסֵף) . וְיֵשׁ אוֹמְרִים דְּמִכָּל מָקוֹם לִפְנֵי רַבּוֹ מַמָּשׁ אָסוּר (ריב''ש סי' רע''א), וַאֲפִלּוּ שֶׁלֹּא בְּפָנָיו מַמָּשׁ, אִם הִתְחִילוּ בִּכְבוֹד הָרַב לוֹמַר שֶׁיִּשְׁאֲלוּ לָרַב, אוֹ שֶׁהָרַב מֻפְלָג בְּחָכְמָה וְזִקְנָה, אֵין לְהוֹרוֹת בְּעִירוֹ. (בְּבֵית יוֹסֵף מִגְּמָרָא בפ' הַדָּר) .
A Posek in his own right may pasken even in proximity to the local Rov. (Unless he's close to his Rav Muvhhak, according to some opinions.) Another loophole may be to start off answering that "one should ask the LOR" and then actually answer the question.
וְיֵשׁ אוֹמְרִים דְּתַלְמִיד גָּמוּר תּוֹךְ י''ב מִיל חַיָּב מִיתָה, אִם הוּא מוֹרֶה. חוּץ לְי''ב מִיל, פָּטוּר אֲבָל אָסוּר.
Some say that a student (without any Smicha, I assume) may not Pasken anywhere, ever.
הגה: וְיֵשׁ אוֹמְרִים דְּדַוְקָא בְּרָגִיל לָבוֹא לְעִירוֹ שֶׁל תַּלְמִיד. אֲבָל אִם אֵינוֹ רָגִיל לָבוֹא שָׁם, רַק בְּדֶרֶךְ אַקְרַאי, מֻתָּר, כָּל שֶׁהוּא חוּץ לְג' פַּרְסָאוֹת (מָרְדְּכַי בְּשֵׁם ריב''א) .
Some limit this to the case when the person's Rov is close by or comes to visit frequently.
תַּלְמִיד חָבֵר תּוֹךְ י''ב מִיל, פָּטוּר אֲבָל אָסוּר. וחוץ לְי''ב מִיל, מֻתָּר. אַף עַל פִּי שֶׁנָּטַל רְשׁוּת מֵרַב אֶחָד, לֹא סָגֵי, עַד שֶׁיִּטֹּל רְשׁוּת מִכָּל רַבּוֹתָיו הַמֻּבְהָקִים.
A student who is on par with his Rov still has the ~12 KM limit, unless he gets permission from all his Rov-Muvhaks. (From here it seems that one can have multiple Rav-Muvhaks. Possibly having learnt most of a subject from each.)
הגה: וְהַאי מֻבְהָקִים לֹא מַיְרֵי כִּשְׁאָר רַבּוֹ מֻבְהָק שֶׁרֹב חָכְמָתוֹ מִמֶּנּוּ, דְּאִם כֵּן לֹא אֶפְשָׁר לִהְיוֹת לוֹ הַרְבֵּה רַבּוֹתָיו מֻבְהָקִים, אֶלָּא ר''ל תַּלְמִיד גָּמוּר, לַאֲפוּקֵי תַּלְמִיד חָבֵר דְּהַיְנוּ שֶׁנִּתְגַּדֵּל בַּתּוֹרָה וְנַעֲשָׂה חָבֵר לְרַבּוֹ, דְּהַיְנוּ שֶׁהוּא קָרוֹב לִהְיוֹת גָּדוֹל כְּרַבּוֹ. (מהרי''ק שֹׁרֶשׁ ק''ע וּבֵית יוֹסֵף בְּשֵׁם רשב''ם פ' יֵשׁ נוֹחֲלִין) . מִיהוּ יֵשׁ חוֹלְקִין וּסְבִירָא לְהוּ דְּאִי קִבֵּל רְשׁוּת מֵרַבּוֹ אֶחָד, מְהָנֵי לְהוֹרוֹת חוּץ לְג' פַּרְסָאוֹת. (תְּשׁוּבַת הָרַשְׁבָּ''א סי' קי''א וריב''ש סי' רפ''א) . אֲבָל תּוֹךְ ג' פַּרְסָאוֹת, לֹא מְהָנֵי לֵיהּ. וְיֵשׁ אוֹמְרִים דְּכָל שֶׁאֵינוֹ רַבּוֹ מֻבְהָק, דְּהַיְנוּ שֶׁאֵין רֹב חָכְמָתוֹ מִמֶּנּוּ, תַּלְמִיד חָבֵר הוּא (בֵּית יוֹסֵף בְּשֵׁם הָרַמְבַּ''ם) .
Here the Rema tries to define a Rav Muvhak and from who one needs to get permission.
As an interim summary: One may be allowed to answer a one-off question if there's no Rov in close proximity.
The Shulchan Aruch continues with these caveats:
ז לֹא מִקְרֵי הוֹרָאָה אֶלָּא כְּשֶׁמּוֹרֶה עַל מַעֲשֶׂה שֶׁבָּא לְפָנָיו, אֲבָל אִם שָׁאֲלוּ לְתַלְמִיד הֲלָכָה כְּדִבְרֵי מִי, יָכוֹל לוֹמַר מַה שֶּׁבְּדַעְתּוֹ, כֵּיוָן שֶׁאֵינוֹ מוֹרֶה עַל מַעֲשֶׂה שֶׁבָּא לְפָנָיו.
To Pasken theoretical cases is allowed; even voting/deciding which Posek to follow - in theory - is allowed.
באר היטב (יא) מעשה. אבל אם מורה על מעשה שבא לפניו אפילו שאלו הלכה כדברי מי אסור לומר כדברי פלוני. ש''ך:
But informing the questioner which opinion one follows is forbidden.
ח לֹא מִקְרֵי הוֹרָאָה אֶלָּא בְּדָבָר שֶׁיֵּשׁ בּוֹ חִדּוּשׁ לַשּׁוֹאֵל, אֲבָל בְּהוֹרָאָה יְדוּעָה שֶׁהִיא פְּשׁוּטָה לַכֹּל, כְּגוֹן נוֹתֵן טַעַם לִפְגָם אוֹ לְבַטֵּל אִסוּר בְּשִׁשִּׁים וְכַיּוֹצֵא בָּאֵלּוּ, מֻתָּר.
"Paskening" refers to issues which the questioner has no knowledge about. But refreshing his memory is permitted.
ט יֵשׁ מִי שֶׁכָּתַב, שֶׁכָּל הַכָּתוּב בַּסְפָרִים מִפִּסְקֵי הַגְּאוֹנִים יָכוֹל לְהוֹרוֹת בִּימֵי רַבּוֹ, רַק לֹא יוֹרֶה דָּבָר מִלִּבּוֹ וְלֹא יִסְמֹךְ עַל רְאָיוֹתָיו לְדַמּוֹת מִלְּתָא לְמִלְּתָא מֵעַצְמוֹ.
Some state that the prohibition against Paskening does not apply to anything clearly written in a Halacha-Sefer. However, it has to be clearly stated in the book; comparing to similar cases using logic is forbidden.
י יֵשׁ מִי שֶׁכָּתַב שֶׁאָסוּר לְחָכָם לְהַתִּיר דָּבָר הַתָּמוּהַּ שֶׁנִּרְאֶה לָרַבִּים שֶׁהִתִּיר אֶת הָאָסוּר.
Some say that [even] a Posek may not permit something that everybody assumes to be forbidden.
באר היטב (יב) התמוה. כתב הש''ך נראה דהיינו דוקא אם מתיר בסתם אבל אם אומר לשואל טעם בדבר ומראה לו פנים או שמביא ראיות מתוך הספר מותר:
...unless the reason for the ruling is clearly stated.
יא לְאַפְרוּשֵׁי מֵאִסוּרָא, כְּגוֹן שֶׁרוֹאֶה אָדָם שֶׁעוֹבֵר עֲבֵרָה מִפְּנֵי שֶׁאֵינוֹ יוֹדֵעַ שֶׁהוּא אָסוּר, אוֹ מִפְּנֵי רִשְׁעָתוֹ, מֻתָּר לְהַפְרִישׁוֹ וְלוֹמַר לוֹ שֶׁהוּא אָסוּר, בִּפְנֵי רַבּוֹ, שֶׁבְּכָל מָקוֹם שֶׁיֵּשׁ חִלּוּל הַשֵּׁם אֵין חוֹלְקִין כָּבוֹד לָרַב.
Despite the issur of paskening in one's Rav's presence, one may prevent somebody from doing a sin - by informing him it's forbidden - even in one's Rav's presence. This is because Hashem's honour takes precedence over one's Rav's honour.
יב אִם בְּנֵי בֵּיתוֹ שֶׁל תַּלְמִיד הֻצְרְכוּ לְהוֹרָאָה וְשָׁאֲלוּ לוֹ, לֹא יוֹרֶה לָהֶם בִּמְקוֹם רַבּוֹ. וְלֹא כָּל מִי שֶׁמֵּת רַבּוֹ מֻתָּר לֵישֵׁב וּלְהוֹרוֹת, אֶלָּא אִם כֵּן הִגִּיעַ לְהוֹרָאָה. (טוּר בְּשֵׁם הָרַמְבַּ''ם) .
Even in one's own home one may not Pasken, if one is in the locale of one's Rav (as defined above.)
Even after one's Rav dies, one may not Pasken unless one is sufficiently learned. (I.e. one could get Smicha, for example.)
Now to your questions: May you answer the following:
"Do I have to say a bracha on this dessert?"
This is the hardest to allow. If it's not clearly mentioned in a "Bracha-guide" book, and it's not a simple case (it's mashed fruit on crushed cookies, not watermelon) then you may have an issue answering this, if your Rav lives close by.
"When's the last time I could daven Mincha today?"
This should be allowed; everybody knows the answer is "until sunset, or shortly thereafter" and it's a matter of looking it up in a Luach.
"Am I allowed to carry this back home?" (on Shabbos).
This may depend on the actual question:
Muktza issues? There are lots of books - even with pictures - helping one define what is Muktza. If it's something unusual (an iPhone case I now want to use to store my Siddur) then it may be problematic.
Hachana - preparing for tomorrow issues? This may depend on the complexity of the question and the knowledge of the person asking.
Eruv issues? If you know the Eruv is OK then you're simply passing on well-known information.
(Now I have to ask my Rov if I got this right.)
There is permission for a non-rabbi to answer in the style of "maareh makom ani lach (I am a source citation for you)." If someone calls me and asks, "I have porkchops in my freezer, can I eat them?" it is not psak halacha for me to say no. I am still within the boundaries of this general permission without citing my sources.
In most cases, there are both questions of law and questions of fact which enter into what the halakha will be. However, sometimes one or the other is already obvious and known to all involved. I once asked a rabbi what bracha to make on a certain kugel. He responded "if it is noodles, say mezonot. If it is potato, say adama." That answered the question of law and not the question of fact. It was unhelpful because I knew that and wanted an answer to the question of fact. Absolutely anyone who knew the facts could have answered with the facts and it wouldn't constitute psak. If you read when sunset is in your local newspaper and use that to calcuate the tme for mincha, the newspaper isn't paskening for you.
If he does not need to know right now then he should consult his rabbi, but your question describes (mostly) immediate cases. Assuming he's not going to call his rabbi before dessert or that he's asking the carrying question on Shabbat (so he obviously can't ask if his rabbi is not present), it seems that you have an obligation to not withhold information and thereby cause him to transgress. On the other hand, you cannot rule for him and should not try. Therefore you should say something like "I have learned halacha X and halacha Y which might bear on this" or "I learned from Source Z that..." without authoritatively answering his question. That puts the responsibility back on him but you have supplied some relevant information.
I don't have a source for this other than observed behavior: this is how people responded to me when I asked such questions, and from that I learned to respond to others the same way.
You are allowed to answer and should answer, so long it is a question you can answer.
As long as it's not debatable you can answer. For example: q:"can I light a candle on Shabbos to look for my bottle of water that I left in my car?"a: no.
Although there are laws requiring a "rabbi" for judgments requiring a court, most regular forbidden vs permissible areas are within the purview of any Jew (eid echad ne'eman b'issurim) There are a number of sources which show this (I'll try to expand later), as an example:
How can a man trust his wife (who has a chezkas tumah) that she knows how to become tahor if she is not a Rabbi.
The only issue would be: Is the question within the experience of this friend.
