Vivat least vaguest.
Join Date: May 2016
Visual inspection of both ends of a cat, is one way. Else you need to check pressure before and after the cat when you are pushing the car, just idle or light loading will not give an accurate result. You should be easily able to see through a cat with a bright light on the far side of it.
A dying lambda sensor may not cycle around its typical designed operating voltage on a fully warmed motor, or you can put it on a scope. Else use a gas analyser or third party lambda sensor temporarily hooked to the end of your car's exhaust pipe.