Familiarity is a good clue, but the difference is really just informality, which you can also have with strangers especially if there is any sense of solidarity

Familiarity is a good clue, but the difference is really just informality, which you can also have with strangers especially if there is any sense of solidarity. If you meet someone who is very low-key and egalitarian like an old hippie or punk or the like, you may actually alienate them with "fake" responses because it is fake and not open, and might imply that you perceive a status difference. But even with a status difference, someone working as a valet could say to a somewhat uptight and wealthy customer "hot day", or similar, and if the customer is not totally mean and uncool he will respond pleasantly to this and the formality level will be a bit reduced. But someone who thinks he is high-status probably doesn't want to hear a long paragraph about the valet's life, because that guy is an asshole. It is true that a man shouldn't complain to people that his wife is on her period without extreme caution at best. In the same way, I cannot go around using "du" or "tu" with everyone I meet either and "too much information" is a thing almost anywhere.