theoretical scenario...
a baal teshuva (someone not raised observant who later in life became observant) wants to get married. he doesn't have a copy of his parents kesuba.
What bothers me specifically about this situation is how can a person be counted for a minyan but then be told we don't have sufficient proof of your Jewishness to marry you.
The Gemara says (Yevamos 47a):
ת"ר מי שבא ואמר גר אני יכול נקבלנו ת"ל אתך במוחזק לך בא ועדיו עמו מנין ת"ל וכי יגור אתך גר בארצכם אין לי אלא בארץ בח"ל מנין תלמוד לומר אתך בכל מקום שאתך אם כן מה ת"ל בארץ בארץ צריך להביא ראיה בח"ל אין צריך להביא ראיה דברי ר' יהודה וחכמים אומרים בין בארץ בין בחוצה לארץ צריך להביא ראיה
The Sages taught in a baraita: With regard to someone who came and said: I am a convert, one might have thought that we should accept him; therefore, the verse states: “And if a convert sojourns with you in your land, you shall not oppress him” (Leviticus 19:33). The emphasis on “with you” suggests that only someone who was already presumed by you to be a valid convert should be accepted as a convert. If he came and brought witnesses to his conversion with him, from where is it derived that he is to be accepted? It is from the beginning of that verse, which states: “And if a convert sojourns with you in your land I have derived only that a convert is accepted in Eretz Yisrael; from where do I derive that also outside of Eretz Yisrael he is to be accepted? The verse states “with you,” which indicates that in any place that he is with you, you should accept him. If so, what is the meaning when the verse states: In the land? This indicates that in Eretz Yisrael he needs to bring evidence that he is a convert, but outside of Eretz Yisrael he does not need to bring evidence that he is a convert; rather, his claim is accepted. This is the statement of Rabbi Yehuda. And the Rabbis say: Whether he is in Eretz Yisrael or whether he is outside of Eretz Yisrael, he needs to bring evidence.
And the Rambam Issurei Biyah, 13:10 says:
אֲבָל מִי שֶׁבָּא וְאָמַר שֶׁהָיָה עַכּוּ"ם וְנִתְגַּיֵּר בְּבֵית דִּין נֶאֱמָן. שֶׁהַפֶּה שֶׁאָסַר הוּא הַפֶּה שֶׁהִתִּיר. בַּמֶּה דְּבָרִים אֲמוּרִים בְּאֶרֶץ יִשְׂרָאֵל וּבְאוֹתָן הַיָּמִים שֶׁחֶזְקַת הַכּל שָׁם בְּחֶזְקַת יִשְׂרָאֵל. אֲבָל בְּחוּצָה לָאָרֶץ צָרִיךְ לְהָבִיא רְאָיָה וְאַחַר כָּךְ יִשָּׂא יִשְׂרְאֵלִית. וַאֲנִי אוֹמֵר שֶׁזּוֹ מַעֲלָה בְּיוּחֲסִין:
Someone who comes and says that he was a non-Jew and converted in court, he's believed, as the same mouth which said that he's forbidden said that he's permitted. When is this true? In Israel in the time when the assumption was that everyone is Jewish. However, outside of Israel he has to bring proof and only then marry, and I say that this is because Yuchsin is strict.
(emphasis mine).
Practically, there are a few Vaads which provides a service to prove Judaism.
My parents didn't have a ketuba either. so when it came time to marry, I went to a Beis Din with birth certificates for myself, my mother and grandmother, as well as the civil marriage certificate (not ketuba) from my great-grandparents.
The Beis Din asked me some questions and I might have shown other documents, I forgot by now. But the Beis din will tell you what they need to issue you a proof of Jewish status letter. The Beis Din certified that I'm Jewish in this letter, and I showed it to the Rabbi who married us. This is standard procedure for those who were born Jewish, but came from secular or assimilated homes and as such don't have a valid ketuba from their parents.
By the way, one of my siblings got the same proof of Jewishness letter from the Beis Din where I received mine, and used it to prove their Jewish status to the Israeli Rabbinate so they could get married in Israel.
