Some are obviously males at just a few weeks old, with huge combs. Partridge and buff colors may have a darker shade of red/gold on the shoulders/wings. Some keep you guessing until they start crowing, which might not happen until 8 months or so.
Silkies can be very difficult.
X2 on both of junebuggena's posts.If comb development doesn't give them away by 3 months old, you'll have to wait for the adolescent cockerel behaviors. Those include crowing, fighting intensely with other suspected/known males, dancing and trying to mount females.