Cat performance is sometimes determined by using an infrared thermometer to compare temps at the cat inlet vs the cat outlet. The exhaust gasses should significantly increase as they pass through the cat. ie: something like 600 deg F at the inlet and 1200 at the exit - just guessing from memory.

Also, some of the generic code readers will give you real time O2 output voltages while the engine is running. So, if all is well, you would see the upstream sensor voltage fluctuating and the downstream sensor holding fairly steady.

Not that it would help your situation a whole lot, but here are a couple videos that help to explain catalytic converter testing and some do's and don'ts. The second one is about an hour in length but is quite informative - like talking about how to "break-in" a new cat and the difference between the size of holes in the ceramic honeycomb "bricks" in aftermarket vs OEM cats.

https://www.youtube.com/watch?v=9VZ5K8n5jj0

https://www.youtube.com/watch?v=OwPaPUtGFZc