With my leghorns last year, it was very noticeable by three weeks, maybe sooner (I just have pics at three weeks of age to prove it) that there was a difference in the combs. I had 6 roosters out of my 15 chicks.
This year I have 10 RIRs and black australorps (10 in all) and I ordered all pullets. They are 5 weeks old now, and no sign that there is a stray roo in there based on the combs.
I'm sure breeds are different....but with my leghorns, it was obvious quite early which ones were the roos!