Egghead, Jr is correct. There are indicators, however, a small percentage of BR chicks just don't follow the rules very well, LOL. And I've noticed that my heritage line of BRs is a bit more difficult to sex than the original hatchery BRs I had years ago. Right now, I have an almost 7 week old BR chick that I was sure was a pullet, though her coloring was definitely more what folks would say is cockerel-coloring. The comb/wattles are small and barely pinkish while the two known cockerels were obvious for weeks already. So, I may have to rename that one and sell it off if it's a slow maturing male.
My very first hatchery BR hen had cockerel-like traits and fooled a lot of folks on BYC for weeks. So, though there are the indicators that the previous poster mentioned, and they are usually pretty good to go by, taken all together, but sometimes, they will lead you astray.
Most of my heritage line males have some dark wash on the legs, which is normally a pullet trait, for example.