Well their "comb type" is (Correct me if i'm wrong) crested. And the bigger the crest the more likely it is a rooster. When its moderate it's probably a hen/pullet. Also the comb would be bigger, and the wattles a lot more developed.
they have walnut combs and the wider the comb the more likely it will be a cockerel. silkies are slow developers so you cant really go by comb and wattle development. best bet is to wait til it crows or lays. the crest can also be used to determine sex pullets will have nice rounded crest while cockerels will have a crest with streamers (feathers that are longer than the rest sticking out from the crest) but thats usally around 5-6 months of age if not later.