There are various methods to determine the usefulness of catalytic convertors. One that lends its self to a non shop setting is this: run the engine at idle and measure the manifold vacuum, then raise the engine speed to 2000rpm (unloaded) and measure the manifold vacuum. The vacuum readings at 2000 should be greater.
I have only done this on good cars as I only read it on iATN last year. I at first doubted that the vacuum would be higher. I was wrong. Catalyst restriction is not a yes or no answer though and I personally am acquiring a data base (experience level) on good engines and then hopefully on some plugged ones as we see them.
I have seen them so plugged that if I could get to 2000 I would have had zero vacuum. I have also seen them so plugged the car wouldn't run.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician