Here's the general guidelines:
- Most props have the size molded in. The numbers face the front of the plane, no matter if the motor faces front ("tractor" configuration) or the back ("pusher" configuration).
- If you cut the prop in half, it will look like an airfoil. The concave "undercambered" side faces the rear of the plane. Think of it as an airfoil (which it is!) pulling the plane forward [diagram from rcgroups].
- If you feel something "wrong" with the thrust and the electronics seem fine, land and check your prop. There's a good chance you've got it backwards.
jj604 sums it up nicely:
'Why the hell they didn't just mold a big "FRONT FACE" and "<- ROTATION DIRECTION" on the prop is beyond me.'