Silkies are known for taking a long time to tell gender. Part of it is the dark skin, part of it is the unusual comb. The only thing you can really do is watch the combs. Small, flat combs are what you want in pullets. Combs showing significant height or that look swollen before, say, 14 weeks probably indicates a cockerel.
Many people choose to send in feather samples to a lab to get the gender of their Silkies earlier. It costs money but isn't outrageous.