There is a concept from the Gemara referred to as "Puk Chazi", go and see (as I understand). What are the applications and qualifications of this idea? Does "Puk Chazi", i.e. noting what people typically do, only apply when there is an open question, or are there sources which indicate that it has the strength to settle a disagreement among poskim?
Probably depends on who applies it, and how.
My impression is that if a posek is faced with a difference of opinions among earlier decisors, and he feels that both sides have merit, and clearly normative practice has won out one way, the "puk chazi" principle gathers a great deal of weight.
Here's Rabbi Yehudah Herzl Henkin's wording in Tradition 37:3 (2003)
Widespread practices influence rabbinic rulings in a number of ways, including:
1)“Go see what the people say.” In case of doubt as to what is halakha, custom is decisive in choosing between various options.