you take an ohm meter and put it acrost the poles. It should give you a high resistance reading, and as you keep the probes on the poles it should get more and more resistance until it goes to infinity because of the way caps work.

But you're talking about jumping when the Tx is on and the motor is off right? That's probably not gona be the problem.

here's what I would do. Disconnect everything except the receiver battery or BEC, and then start adding things one at a time and see if you can isolate the problem that way.