I've always had terrible luck getting accidental cockerels with my pullet orders. So I am always breathless each day when I examine combs for that reddening that indicates I have a future roo. But always the reddening occurs before the eight week mark. If combs remain muted, no matter if yellowish or pale pink, I figure I'm home free. Also, wattles, on breeds that have them, will begin to be apparent on the boys around six weeks.
I did give myself a scare a few weeks ago when my present four were between seven and eight weeks old. Each of my four EEs had a different hue on their tiny combs. A couple were more deeply pink, one a very pale pink, and one had a blackish nub for a comb. But none have reached the unmistakable red that would indicate a cockerel.
Another surefire way to tell if you have a cockerel is to part the saddle feathers and look at what color pin feathers are emerging. This is only a good test with chicks from a breed where roosters are multicolored. You should see bright, contrasting colored pin feathers on backs of cockerels as they approach seven or eight weeks.