Well, usually, the males get shoulder markings that the girls do not have. When the tails grow out some, you'll see black feathers on the females, edged in white, but barring on the males' tail feathers. In this current batch, I had a couple of veeeerrrrry slow developing cockerels that I mistook for pullets. Mine are 7 week old now, but most of them were pegged as what sex they were by four weeks old.