From what I've heard, you can't really tell anything until they crow or lay an egg! It's probably the most frustrating thing about raising chicks!! There are lots of ways to tell, but since there are always exceptions to any rule, you can't count on them. From what I've learned in the past three weeks:
Roos get their combs and wattles earlier than pullets. They also turn red sooner.
Pullets feather out faster.
Roos tend to have thicker, heavier legs.
Nothing is fool-proof. If you read around on this forum, you'll learn a lot about sexing birds...some of it is probably true and some of it is just old wives tales that sometimes work and sometimes don't.
Good luck and enjoy your chicks!!! And