I can usually tell by about 6-8 weeks depending on how red their face gets. I think by three months, you should know for sure, unless they crow earlier. I have week old chicks with my month old chicks right now, and they are doing fine. The youngest are fiesty little things lol
I usually judge mine on feather growth (especially in the tail area) and stance. Two and three look like pullets, and one might be a pullet too, but that comb is awfully large. Sometimes the girls will get a pretty large comb, so if it stays yellow, you should have a pullet. If it's red or...