Depends. Some breeds are bred as sex links and you can tell as soon as they hatch. Production birds have been bred by large hatcheries to be able to tell by their wing feathers, at a day or 2 old, which is cockeral and which is pullet. Then again you have specially trained "vent sexers" that are highly skilled and usually trained for many years in Japan on sexing chicks at 1 day old by pushing on the area above the vent to see the sexual organs (which are on the inside). Only the very large hatcheries can afford these experts though since they are paid very well for these services. Even then they have a 10% error rate.
Some breeds have different colored feathers that show up in a few weeks and others show hackle and saddle feathers fairly soon and some... well..., lets just say they are pretty close to crowing or laying by the time I can figure it out for sure.
Post pics and name the breeds you have if you want more specific info. Somebody here will likely have had experience with the same breeds as you have.
