Nicole is right, the little roo will have a larger and reddish pink comb compared to the pullets. I've also been able to tell if I have a roo or not by looking at the back of the leg see if there is a little round nubb about ankle high, that showed up early with my chickens at least when I noticed at about 8 weeks I'd say. Maybe I should go check my brooder now, after all I have 52 four week old chicks in there...I ordered 1 cockerel but it looks like I may have a few in there.