Here's a trick that might work.

1. Take off your back wheel. Lay it horizontally across a waste can with the cogs facing up.
2. Spin the gear cluster with your fingers. Notice what part spins and what stays still.
3. Shoot some WD40 into the crack between the moving part and the stationary part. Let it soak in.
4. Spin the gear cluster with your fingers again. If you can get it to move one way but not the other you're good.