Originally Posted by upallnight

Now wasn't that easy??? If you have a scanner that can monitor the PID'S for the rear 02 sensors you can compare the voltage that the sensors are putting out. Rear 02 sensor will put out almost a constant voltage whereas pre-cat sensor voltage will be going up and down to adjust the fuel mixture.

Some time an OBDII diagnostic will provide more info than BMW specific diagnostic. You can just purchase an inexpensive Bluetooth scanner and use an App such as OBD Fusion or Torque to view live data.