At what age? If you want to tell the sex at hatch, if it has a depression on it's head where a comb would be, it is a roo. If it has either a smooth head, or a thickening where the comb would be, it is a pullet. This sounds crazy, but I have done it alot on mine, and it worked each time! If they are older than just a few weeks old, then you're outta luck. You can start looking for spur buds.
It really seems to be difficult to sex these guys at an early age. I spend so much time with my chicks that I have a fair accuracy rate from observing their behavior, interaction with other chicks and the way they carry themselves and react to various situations. The young chicks that I predict to be roos usually are but then there are always a couple that I just can't figure out until they crow or lay an egg. Good luck with yours!