PDC Sensors use echo location technology, so you can hear a clicking sound from working ones, the sensor that does not emit a clicking noise is the faulty one, saves swapping them..

Desperately looking forward to finding out how to remove the sensors.. I have one as well. Please can someone update on how to remove a PDC sensor in the rear, as I only just got the car 2 days ago and dont want to drive in case i hit something, feels like I am driving bigfoot!