Depends... normally at a few weeks the roosters tend to have grown large, red combs. It is normally around 2 months you get a good idea, and four a definate. good luck!
When if it crows, it's a rooster. Lays an egg, it's a hen.
Basically, it's hard to tell on many of them and most people make educated guesses. I've heard pointy hackle feathers means roo and rounded hackle feathers means hen. Early comb and thick legs: roos. But gosh, there have been tons of birds that have stymied the experts.