Three weeks has always been my magic number. Within one day of their three weeks "birthday" the roosters combs pop out. If you have a rooster, it's easier to tell because you will look at everyone, but the rooster has a slightly larger comb.
If you look at this photo you will see three peeps peeking out from under mama Marbles. They ended up being a one of each. One Delaware, One Astralorp and one Barred Rock/Delaware cross. The yellow peep on the left and the center peep are boys, and the peep on the right is a female. If you look closely at the difference in the combs you will see the difference and know what to look for at the three week window.
I had a batch of 12 barred rocks, and on the very day of the third week I was putting them outside in their little run-pen. I was in flip-flops and they were running around at my feet. One started to attack me and would pinch my toes and twist. It finally drew blood and I had to get out of the pen. I sat down and was putting some yarrow/plaintain on the bleeding toe when I noticed that one had a bigger comb than the others. That's when the three week window became apparent to me, and it has been that way with all other peeps since then. And, yes, that little one grew into a rooster and had to be dealt with before he even crowed. He was "something else".