Yeah I hate the "how can you tell them apart " question too!!
My pet peeve, my IL's used to make it a game everytime they saw them to guess who was who, " no dont tell me, Ill get it in a minute"
My BIL still says to them, I have no Idea who's who, tell me,, whats your name, AND one has had her hair cut short ( coz she was sick of being a twin)