It's hard to tell. I'm much better than I used to be, but 2 roosters slipped into my new layer flock, anyway. That's okay, we ate 'em.
Since I butcher, I can tell you that the definitive feature of the rooster is his testicle sack, which is located just inside the vent, and by the tail. You can feel for that and know for sure. The other identifying feature is the saddle feathers, which start at the base of the tail feathers and frame them. Mature roosters have glossy, or, oily feathers, too.
Different breeds are easier to tell apart from each other. When I had RIR's, the roosters grew prominent combs and waddles, and the pullets don't grow them until they are ready to lay, and the size difference IS noticable.
I had a rosecomb RIR(I think) rooster this year, and he didn't really Look like a rooster until he was mature.
Just in case it happens, it's okay to keep more than one rooster together, as LONG as they were raised together. Otherwise they will fight and injure each other, if not fight to the death.