i have been lucky to be able to tell by combs, and behavior (if given space to act naturaly and start to show some personality), at at least three weeks. the females, if you have both, will have a thinner shorter, starting to get slightly warped comb, while males will have straighter, taller thicker combs. course i havent raised up any of my rosecombs to that age, so only know works for common combed breeds. also males will start to spare, but female wil be over all dominant and eat and push in and be more our going especially to inspect food, while roos will hang back and let females eat after investigated. females are all round more aggressive also, despite said about roosters sparing. though i have one hen who crowed, and spared, and acted just like roo, but then she mo;ted out and started laying... lol so there are exceptions!