Wait a while. Eventually they will either crow or lay eggs.
If they crow, it still MIGHT be a pullet. But if it lays eggs it definately is....
Seriously. Unless the chicks are sex-links (bred specially so you can tell gender either by feather length or color) the only way to tell gender in young chicks is to vent sex. That is a method that takes a lot of time and training to pefect, and then it's still only about 90% accurate.
In a lot of breeds, you can tell the difference between the genders by 4-5 weeks. Baby roos will have much more developed combs and wattles and they will be much redder.
Silkies and their kith and kin are still pretty hard to tell apart many times until they are much closer to sexual maturity. I have some silkies I hatched out back in November and I am just within the last couple weeks feeling pretty confident of gender. I had hunches early on, but wasn't CONFIDENT until recently.
You can always post pictures in the 'gender/breed' category here on the forum and get the expert opinions of those with more experience.