Where's that bad cat?

[NewArt]

P0430. Bad cat bank 2. If it is indeed a failing catalytic converter ( and I am not sure of this as we all know that sometimes these pesky codes are cryptic and mean that it's really somebody else's fault), which cat is to blame? Logically it would be the pre-cat as the O2 sensors are found fore and aft of that one in the model that I have. If the main cat is monitored, I don't see how.
Having found a nasty fouled and corroded O2 sensor on the aft side of the suspect pre-cat (since been changed), i fear that an oil overfill might have caused some damage.
So essentially, I would like to find a method to test the cat(s) to see where I should throw my money next!
Cheers!