If it is a chick, you usually have to wait till they are around 3-4 months old before you can tell. At 18 weeks old, the girls (pullets) will start to lay eggs, and the roosters will usually crow.
At 4 to 6 weeks old, alot of breeds have the secondary sex characteristics start to show up and it's possible (but not definite) to tell the difference. I agree with Harley's Girl. Post some pics and let us see if we can help you.