[Dazzler]

If you hold the spur and turn one wheel, the other wheel should rotate in the opposite direction. If you hold the spur and one wheel, and try to turn the second wheel, there should be a good amount of resistance before you can make the diff slip / second wheel rotate.

Also, when the wheel does turn, it is useful to identify whether it is the diff which is slipping or the actual slipper on the spur. You'll obviously need to have the spur gear removed to hold the spur, so you should see if the actual slipper is slipping..