Silkies are on the hard side to sex.
For silkie roosters, they often have tsome longer feathers sticking out of their crest, like this:
Where as the hen does not:
Also, roosters have bigger combs than the hens, though not usually as large as this ones:
Where as a hens comb usually looks like:
You can also tell by the earlobes, but I can't remember how.
Polish are much easier. Since they have normal feathers the males will have the saddle feathers, which are the long narrow feathers that hang down just infront of the tail. Their tails also are longer and have feathers that curl back. The main difference is their crest. Polish males have longer feathers that flip all over the place, kind of looks like fireworks going in different directions:
Hens have a more compact comb and it should form an almost perfect, full circle, however the pet quality hens may have fewer feathers so they'll be more wild and roosterish looking. They dont have long pointed saddle or neck feathers, and their tails go straight up and dont have any feathers that curl back down. Here's a picture of one of my hens: