you will either need to scope the sensor or find a scan tool that will show you sensor waveform patterns...
WHEN COLD, the pre-cat (B1S1) and post-cat (B1S2) sensor waveforms will be almost exactly the same in both frequency and voltage swings (amplitude is probably the right word for that).
After driving around and getting the cat hot enough to "fire off" the waveforms will change. B1S1 will still be relatively the same as it was cold, but B1S2 will be at a somewhat more steady voltage (NO voltage swings). it won't look anything like the other waveforms.
you can rule out the rear sensor by that method. if the rear is doing something weird compared to the front during cold running, it's likely a sensor fault. If after driving, the waveforms are still fairly similar, the cat is not operating correctly.