Once they are around 6-8 weeks the males will start crowing. That is the most definite sign. Another thing is to look at their saddle feathers. The boys have longer, prettier, pointer saddle feathers, while the hens have pretty rounded ones. The neck feathers will be the same. Roosters will also more likely have florescent long tail feathers that are very arched compared to the hen, although depending on breed, not all the time. Roosters will also stand differently than the hens--very tall and straight. If they find treats they will pick it up in their beak but won't eat it right away. They will make a repeated clucking sound and the hens will run over to them and then the rooster will usually let them take it right from their beaks. All in all, it is hard to tell what gender they are when they are this age, so for now you will just have to wait a little while.