i don't usually get asked "is that fake?" i get asked "is that real?" and i just laugh, give them a quizzicle look and say "of course it is! everything i carry is real." but if someone said "that's fake" then yes, i'd go off. lol.
this one girl i never liked once asked me "where did you get that, it's so cute!" i said, "well, at the louis vuitton store of course" and she goes, "oh, it's real?" and i said, "of course it is!" with a big fat smile. lol.