I found this useful...
Just check for voltage where the heavy lead leaves the solenoid that leads to the motor. The motor uses three leads to power it... ground, forward, and reverse. See which solenoid clicks when you select the forward position on the controller and measure the voltage from that solenoid as follows... check the voltage on both sides of the two bigger solenoid posts. One of those leads comes from the battery, the other lead goes to the winch motor. Measure where the lead leaves to go to the motor. If the voltage is on just one of those two bigger solenod lugs (the two smaller leads control the solenoid) and the solenoid is 'on', the solenoid is bad. If there is voltage leaving the solenoid going to the winch motor, odds are the winch motor is bad.
Thanks worldwide interweb!