As you can see, visually detecting the gender of young chicken is an art, not a science. If it's really, really, really important to you to find out the gender of your birds right away, you could spring to have DNA testing done on each one, like they do with parrots. Otherwise, time will tell.
I gave away a d'Uccle at six weeks because I was absolutely positive it was a cockerel. It had a huge, red comb in comparison with two other d'Uccle chicks from the same clutch (who did in fact turn out to be pullets), and its comb looked just like the other two d'Uccles in that clutch that I pegged as boys. Guess what? Yup. It turned out to be a pullet. The other two that looked just like this one at that age were in fact roos. Go figure.
I had a little cockerel who was beginning to crow at 4 weeks (and chase the girls, too). I think he was a pretty early bloomer, though. The sound of a little cockerel trying to crow didn't sound like a sneeze; it sounded like a squeaky door hinge. If one of your chicks is sneezing more than occasionally, take a look and see if you can see crusty or goopy stuff on its eyes or nose. You might have the beginnings of an upper respiratory infection on your hands.