Look at the combs developing. The males tend to develop quicker when looking at combs. Starts being obvious after you've watched chicks grow several times.
Males will also be braver (stupider) and dash about exploring while the females are hanging back or following. Until they are 6 months, chickens have 3 sexes. Male, female and no clue. So odds are that you have one of each.
After 6 months, you will have 2 sexes. About to lay an egg and gumbo. Trust me on the gumbo. Having a yard full of crowing cockerels will make everyone want gumbo.