The best way to determine any faults would be to use a professional level scanner like INPA or BMW Scanner 1.4. In a pinch, you can turn the car on and press the PDC button to activate the system and place your hand over each sensor listening closely. You should hear a faint clicking coming from operational sensors. Obviously any faulty sensors will not give any clicking feedback.
I do recommend scanning the PDC system and it will tell you exactly what is wrong.
