Sarahhaven - are you asking how to tell the difference when they are young or when they're older? If older, then advice given is correct, but if when they're young, it can be difficult, depending on the breed. Some chicken breeds are "auto-sexing" which means you can tell, even as soon as they're hatched, by their markings (e.g.Welsummers), but with others, for example Light Sussex, you have to wait till they're a few months old. Generally, the boys will start to show a longer tail and a larger comb (and they'll crow!!). Sometimes, you can see the spur buds on the legs at quite a young age (cockerels have them, hens don't), but agin, this depends on the breed. If you have several youngsters together, the lads will often start to "spar" with each other, which will also give you a clue, even though there's no obvious difference in the sexes at this age. Hope this helps. K