If you removed the motor, you should see two wires running to it. Connect these two wires to a 12v battery (Do not use your ski battery unless it is disconnected from the ski!), doesn't matter which one you make positive or ground. Motor should turn. If you reverse the wires (switching positive and ground) the motor should turn in the reverse direction.
If you can, use a multimeter on the wires at the same time and see how many amps you are drawing. When it first turns it will max out just under 4 amps but once running should not draw more than 2 amps. If it is drawing more, your gears are binding and you will either have to replace the motor or if you are up to it, tear it apart and attempt to clean, although you are not guaranteed to be able to fix it.
Same if it doesn't turn. You can open it up and attempt to free it up, but most likely it is shot.