The easiest way to learn that you have the correct gear mesh is to use a piece of paper and fit it between the spur gear and pinion gear as you tighten the motor mount screws. After the motor is completely tightened, the piece of paper should be impossible to remove except by turning the spur gear to rotate the paper out of place. With the paper gone, turn the spur gear with a finger and feel the amount of movement or "play" there is between the spur gear and pinion gear. There should be very little play, but you should feel a little bit. This is the correct gear mesh. With practice, you will no longer strip any spur gears. 

Make sure the motor mount screws are very tight, a big crash could move the motor towards or away from the spur gear, making the spur gear strip out.