I think the only way to have a clue is to compare with photos of chicks the same breed and age, because it seems to vary a lot with breeds.What about the feathers vs comb argument? Has it got any merit?
With two hatches of backyard mutts, the males were actually the ones I had no doubt about from about a week of age. Every chick I hesitated about turned out to be female. Even though they had : big red crests and wattles for Laure, huge feet for Laure and Lulu, early wattles before two weeks for Lulu, Mélisse and Laure, and feathers that looked lile sickle feathers for Piou-piou.