If you showed me any of these chickens on their own, I wouldn't know if they were roosters or pullets, but when you see them next to each other, its easy to pick the difference. The hens have less developed crown and wattles compared to the roosters, they are also slightly smaller, this is apparent at a few weeks of age, as soon as they start developing crowns. If you can't see any difference between your chicks, you may have all of one sex, which does make things difficult and you have to start looking at tail shape to figure out the sex.
We don't always get it right and sometimes add a rooster into the pullets (just over-optimistic at the number of layers I think), so we will just keep an eye on them now and see if I counted correctly :)
|A young Rhode Is Red rooster|
|And a Rhode Island Red pullet at the same age|
|A young White Leghorn rooster|
|And a White Leghorn pullet at the same age|