I think it varies with the breed, but I can tell by around 12 weeks for certain. There are breeds where the roosters do not get those and are "hen-feathered".
Thanks SpeckledHen... I have a barred rock, buff orphington, EE and Black Australorp that I'm still trying to figure out what they are. I'm pretty sure the BO and BA are both hens and the BR is a Roo, but the EE has me baffled. I think "he" might be growing roo tail feathers, so I'm thinking roo. Don't you just wish they hatched with a sign saying "roo" or "hen?" Boy that would make things a lot easier!
Have you posted a picture of your Barred Rock? If you have no other BRs to compare it to, you may be wrong about it being a cockerel. Pics showing the entire body from the side where you can see the neck, back and comb areas would help.
when I got my EE roo...the shelter said he was a she!...he had beautiful colors and being a novice, I didn't question it.
fast foward to everyone saying on byc that she was a he! at that point I'd already noticed he'd taken a growth spurt UPwards compared to the girls....and had I known about hackle and saddle feathers, it would have been obvious...along with the long tail feathers that were just starting to bend at the tips!
so.,, I gues...keep an eye on his/her legs too...if they get really long...it's a boy!