With Silkies I find it a bit harder too tell but, their are a couple ways you can tell what gender a chickens is.
1. When they are about 4 months old, the roosters will get silky feathers on their necks.
2. At about 5 months or so, a rooster will grow spurs, but at this age you can only see bumps. Although if you see bumps that are larger bumps then on some of your other birds it probably a guy.
(Then the really obvious stuff)
3. At about 9 months - 1 year of age the bird will start crowing.
4. He'll start mating.
I haven't much experience with silkie roosters, but I hope this helps!