Silkies - Look at the feathers on the chick's head. Males tend to have feathers that stand upright and curve towards the back, while the female head feathers tend to form in a rounded feather puff.
Look at the comb when it develops within two to three weeks of the chick's birth. The comb is the flap of tissue at the top of the silkie's head, and a male will have a larger comb than a female.
Compare chicks from the same clutch. Males are significantly larger than females, and this can be obvious a few days after hatching. This isn't considered a certain method of sexing though because you may just have a large female or a small male. It's also a poor method if you are trying to compare chicks from two different genetic lines.
Listen for crowing. The chicks will start losing the fluffly feathers around four of five months. At that time a male silkie will start attempting to crow.
Look at the saddle feathers just before the tail and the hackle feathers on the neck. These feathers will be long and sharp on a male and gently rounded on a female.
Pekins - At 4 weeks Peking chicks will start to develop the comb (this is on the top of the head). It may not be visible at first but can be felt with e light touch. The females will be yellow and the males red. In time they will go red but he females will always be smaller than the males. You may also see the deveopment of wattles. Red dashes will start to apprear under the beak. Again both males and females have wattles but he females are smaller. Female chicks tend to feather up more quickly and have bigger tail feathers.