The only thing for sure, and for me its a dead give away is...the full comb and wattles. Especially at 16 weeks. Now about the crowing, every thing I've read says that between 16 and 20 weeks crowing means that they were matured and could mate. With that said I feel that this breed has a good % of early crowers, yours will in time. The tail feathers are certainly different by now also.
This is my GLW 16 week old cockerel(on left). I am assuming that he is the only male GLW I have since none of the others have hackle or Saddle feathers like he does and his comb and waddle are huge! He hasn't crowed yet! When they were younger he was one of three suspects but now I couldn't even tell you who the other two were!
So the question I came up with is do all GLW Cockerels have this coloring or could some of the ones that look like the pullets be boys too?