No Power

Sometimes if the cat breaks up, a chunk of it could block the pipe. If you are in neutral and give it some gas you might hear an unusual 'Hiss' from the exhaust.

Taking the exhaust loose in front of the cat and a short test drive might also verify it.

Unless you are kinda 'exhaust capable' and don't wanna waste any more of your time, the EZist thing to do is have a muffler shop verify your suspicions, They'll probably do the work anyway!

But

Before you commit to just any shop, you might call around and get some quotes, seems they are really proud of them cats!

The Medic

Plugged cat or muffler would be possible. Especially with a rattle. Easy to do an exhaust back pressure test. Just remove the O2 sensor in front of the converter, install the test adapter and an old style fuel pressure/vacuum gauge that reads up to 10-20 PSI. Start the engine. You want to see less than 1.5 - 3 PSI on the gauge when you rev the engine. Any higher and you have a problem. Be sure to test after the convertor by either disconnecting the exhaust or removing the rear O2 sensor if it has one. You don't want to replace an expensive converter when it's a plugged muffler.