How do I tell if my catalytic converter is partially plugged??

Horrible performance, poor emessions. Past that, drilla hole & mount a pressure gauge & compair it to what it would be without the cat.
Or just ditch the cat.

Hi flow cats are cheap, welding is easy - for people 2 scared to learn to weld, most exhaust shops charge about $20-40usd for that kind of work.

T a vacuum guage into the intake, and with no load on the
engine (car in neutral or park, air conditioning off, all accessories
turned off), run the engine at about 1500 RPMs. check the vacuum
in the intake at this engine speed. Then, increase the engine speed
to 3000 RPMs and check the intake vacuum. It should be within 1" to 2"
Hg of the vacuum reading at 1500 RPMs. If the vacuum drops
significantly between 1500 RPMs and 3000 RPMs with no load on the
engine, there's something restricting the exhaust flow. Likely the cat....