Our earliest was 8 weeks old when he crowed, which is a real certain giveaway. Within a week the leader of the hatch is usually (but not always) male and the lowest is female (but not always as some are slower developers). At two weeks more certain, unless all are same sex as our last hatch was. The male combs start to grow at about 6 weeks and at 10 weeks they are obvious, but only if you have something else to compare them with, as again they could be all male or all female. Apparently the female wing feathers grow faster than the males but I haven't noticed. There are lots of pointers, but back to the crowing -the only certain first sign. The hens are pretty but cockerels are really magnificent and we still have one -Frankie, so named because he was and still is a bit of a monster.