There are a few ways to know what sex a chick is.
1st- chick sexing/ which is achieved by basically squeezing the poo out of the chick and looking at the cloaca. (not sure which part their looking at i think its cloaca could be wrong) This method must be done very quickly since the chicks are at risk of infection from their inners being pushed outwards and exposed.
2nd- older sexing/ pay close attention to the behavior of your chicks and watch carefully for any subtle differences in physical growth, (such as thicker legs, darker comb, more pronounced comb, early wattle growth- all indicates possible rooster) I opt for this method since I would feel like I'm squeezing the poor chick to death. And so far I've only been wrong once.