I'm not sure if you mean in the incubator or after they are hatched. If in the incubator, the only way I know of to tell which chick came from which egg is to build a small wire cage and put one over each egg. Put a tag of some mark on each cage.
If you are talking about after the incubator or broody hatch, there are several ways. Toe punch, wing bands, colored zip ties around the legs, maybe those colored plastic spiral bands. You can use food coloring but that can wear off pretty fast since they grow so fast. I've even heard of using nail polish on the toes.