Look at the prop's blade end on and you will notice they have a curved cross section, not unlike the sail on a boat. It's concave on one side, convex on the other (A propeller blade is basically an undercambered wing with a serious degree of washout). When it is properly mounted on the aircraft the convex side should be facing forward.

Actually Jetmech isn't entirely correct. Assuming the hub is the same on both sides, any prop can be used as either a pusher or a tractor (puller) prop because it doesn't care which side the motor is mounted on, so long as it's pointing and turning in the correct direction. With a brushless motor this is as simple matter of reversing the direction by switching wires.