typically in cochins (so i'm told), pullets wings feather in faster than cockerels... but usually by the time they're 4 weeks old, you can see the difference in the cockerels' comb/wattles compared to the pullets'... i usually wait at least that long to decide who's who, before getting rid of any extras. but i'm hanging onto everyone until 8-12 weeks at least to assess how they're growing.