One issue is how a rabbi is ordained (semicha as we know it), which is effectively a professional license.
A semicha such as "Yoreh Yoreh" (to "instruct" in matters between man and G-d, such as food kashrut) or "Yadin Yadin" (to "judge" in matters between man and man, such as a contract dispute) generally conveys a license to pasken -- and even make judgment calls on some gray areas as needed -- on the subjects studied.
Some yeshivas (past and present) had/have rabbinic programs that involve less intense study of halacha, and thus don't convey this license. A rabbi from such a program will probably stick to straightforward halacha.
Now (unfortunately) some people have licenses who don't deserve them; and (fortunately) many people can and do become incredibly qualified and practice, without ever having gone for formal certification. But we tend to look for licenses. (For instance: R' Moshe Feinstein writes that if the town mikva is under the auspices of a rabbi with semicha, a majority vote is required before levying a tax for a new-and-improved mikva. If the old mikva has no rabbi, or the rabbi has no semicha, anyone can demand a new mikva.)
I don't think there's a specific yardstick for determining between an "ordinary rabbi with yoreh yoreh" and a "posek", but most rabbis have a sense of when (and who) to ask. The world-class posek is recognized as someone who has studied and mastered all of relevant Halacha (including in-depth training in many fields of halacha that the average yoreh yoreh program may not cover); understands the real situation of the Jewish community; and can use his judgment accordingly in a practical way (as opposed to someone like R' Chaim Brisker, who was a theoretician). It's said that R' Moshe Feinstein was asked how he became a big posek: "people asked me questions, and I guess they liked my answers, as they came back with more questions."
You will find also find language in responsa (such as R' Moshe's) like:
- I would allow it, but only if R' so-and-so concurs.
- Each local rabbi should make their own call, based on the following guidelines.