Determining Which Parking Sensor is Bad

[Aaron407]

We seem to have a bad rear parking sensor in our 2015 that pops up as a warning every time we put it in reverse. Can anyone tell me if there's a way to confirm exactly which sensor is the problem without a code reader able to tap into the BCM? I've tried using a cell phone mic to pick up the clicks while in reverse but couldn't pick anything up.

Any ideas?