Check to see if you have a small test port on the pipe, just forward of the cat on the passenger side. Remove the bolt and put a test gauge on it. You might have to fashion some kind of adapter to fit an NPT gauge into the metric hole.

Normally the backpressure runs about 3-4psi max WOT with the factory-stock exhaust; 30-50psi when some crap floats around and blocks the exhaust!

The "rolling soup can" noise inside the cat, and the intermittent sudden loss of power coupled with the exhaust note of a 2-stroke dirt bike, were also big giveaways, but the gauge proved I had a bad cat blocking the exhaust beyond any doubt.

If you don't have the test port you could always drill a hole in the pipe and then cover it with a bolt welded on the outside, but that's approaching the 'why bother' category IMO.