The onle sure fire method we've found is looking at the feathers on the rump, just before the tail feathers. On hens they "flow" straight back whereas roosters' sort of cascade down toward the ground. We've tried sexing by color, wing feathers, comb, and once we even had a hen start to crow, but the butt feathers give it away every time. I snagged a couple pictures from the internet to show what I mean.