I have been told that the males' wattles are more rounded, though I don't know how accurate that is. The best way to sex them is by their call. I've also read that the helmets of the males is bigger, but it is such a minute difference it's almost impossible to tell!
The whole wattle thing is not exactly always accuate. This year I had one of my "males" hatch a nest. I swear by the looks of "him" he was a he. Well when I saw "him" on the nest I went to check it out. he got off the nest and stood out in the yard doing the hen chirp. So the sound is the best way to determine