Wow, I'm quickly learning that this is the most common type of thread question. Once you own a rooster it is easy to tell usually, but that one looks too young for me to know. I was reading in another thread that you can lift the feathers on the back of the neck to know for sure, because cocks always have pointed neck feathers, and hens are rounded like a "spoon". Hope that helps.
Do you know what breed these chicks are? Looking at them, I am wondering if they are Red Sex Link chicks. If so, they can be sexed by color; the white one is a cockerel, and the reddish ones are pullets.
If they are cross breeds, they could be Red Sex Links, which are produced by crossing a red gene rooster (RIR, NHR, or Production Red) with a silver gene hen (WR, RIW, SLW, Delaware, Light Sussex, etc.).