There is wing feather sexing between newborn and 3 days. After that you watch the combs, waddles, and feathers. Roos will usually have pointy neck and saddle feathers, hens will usually have rounded. It's not foolproof though, my EE hen has pointed neck feathers and there is no doubt that she is a hen. pea combs and small rose combs can be very late to develop but my Wyandottes that just turned 8 weeks yesterday have enough comb and waddle growth to tell the difference now. My SSH which has a larger type rose comb was obvious a cockerel at 3 weeks old. Red is usually a good indicator although occasionally you come across a breed or variety that is just naturally more red. Then you have to rely on early waddle growth and comparing it to others with the same breed and variety if you have that option. My GL Wyandottes are sexed pullets from the hatchery, compared to my other varieties they all look like cockerels, they are so red, but they have the pullet coloring and rounded feathers.
Even all that said, sometimes you have to wait for full tail feathering, spurs, and/or crowing. Most times if it crows, has a long droopy tail, and has spurs, it's a cockerel. There is the occasional instance of hens with these characteristics. If you are still in doubt you will know when it lays an egg or doesn't :lol