The crest feathers are not uniform in length, and does not have the shape of a well rounded pom-pom as in females. The tail feathers are longer than you usually see in a female and have a downward curl. Also there are several hard feathers in the wings. Roos have more of those than hens. Having pointed out all these roosterly qualities, still Silkies are very difficult to sex, so I could be wrong.
Some Silkies mature very slowly (in my experience, the better the quality of the bird the slower it matures). Many of my boys do not crow until 8 or 9 months. I have a bunch that are about 6 months that are acting more like roos, but have not crowed yet.