Wait a few months till they start to crow, but make sure you're watching at the time, or like me you'll be eyeing up 3 potential cockerals and still not know!!
Alternately watch the way the combs/wattles grow, the boys are bigger, sooner.
You can tell via the colouring sometimes, its quite an interesting study, but breeds where the male and female look alike are quite problematical unless you're experienced.