At about 4 weeks, the cockerels have combs that start to become larger and pinky, usually, but I always look at the legs. The boys' legs seem much thicker than the girls'.
Funny! I came across on old sexing doc that said that it was 82% accurate if you went by this:
pullets usually have a darker color down on the head and back with spots or a stripe down the center of the head.
Also on mine I noticed the females tail feathers started growing out before the males.
I now have a 21 week old RIR hen that started laying this week!!!!
Man, I really wish I did! My girls are driving me nuts, one will skip a day, then the other will skip a day. And since I only have the two girls, I'm having trouble filling an entire order each week unless people will let me fill in with cross breeds or Barred Rock eggs!