Help - how to diagnose a bad catalytic converter?

For those who are still following - after many struggles the P0456 Evap leak has been solved!
Another visit to another shop had resulted in no new ideas. I ended up replacing parts one at a time. Figured I might as well since parts are all beyond their lifespan and seem harder and harder to get. First both upstream O2 sensors, then the evap canister, vacuum tube from air box, vapor purge solenoid, and finally the vapor purge harness. That solved the issue. I found that a spill from the battery must have compromised the underside of one of the lines to where it looked rough and probably had a pinhole leak. At least that's my story, lol. Have driven over 10,000 miles since last August and had a few other things go south (window regulator cable broke - 3rd time now on the same window!; door lock died, window switch....)
The random code for the catalytic converter also has not returned so I am biding my time on that. Counting down to 300,000!!

Edit: new upstream O2 sensors solved the "running rich post cat" P0172 code. So the CEL is off for now!

Yeah, mine had a hole worn through the lower tube where it passed closest to the battery.