Washer motor hums, but won't drain, agitate or spin

My washer suddenly stopped working. It still fills with water, but the motor doesn’t work. It hums when I close the lid, so I don’t think the lid switch is the problem. I removed the motor and visually inspected it: I don’t see any burns or broken wiring. Even when the motor is disconnected from the agitator and the pump, it won’t spin and only hums.

What else can I check? I don’t want to shell out $130 for a new motor just to find that wasn’t really the problem.