At several months of age (depending on the breed), they will grow "sex" feathers that will exhibit their gender. A pullet/hen will have short round feathers at the base of the neck (where the back curves) and again on the rear of her back (where her tail feathers will start). A cockrel/rooster will have long sharp/thin feathers in the same areas.
There are those who can "sex" them for you, but I've been advised against it for very young chicks because it could break the yoke they have recently absorbed from being hatched......I hate to put my babies at risk to serve my curiosity.