It depends on the breed. Some breeds of chickens are sex-link, which means the males and females look different from birth. Some people can look at the wing feathers in newborn chicks and use those to determine male and female. Neither of those work for Silkies.
In other breeds the sexual characteristics are more muted and hard to notice. Silkies are one of those breeds. Male and female Silkies tend to look very much alike far longer than other breeds of chickens. You might not notice much difference for several months. Males tend to get a comb earlier and be a little more aggressive, but since even adult Silkies don't exhibit those traits as much as other chicken breeds, it can be hard to notice.
Just remember, usually chicken genders are about 50-50, so if you have four chickens, probably two are pullets and two are cockerels.