Testing for a rattling cat may not be scientific, neither are many other automotive diagnostic tests.
Start with the rattle test, a broom handle etc work fine. (we know what Mark has handy in his garage
). If that doesn't yield any results, then it is possible to have your exhaust backpressure tested and that is the proper way to test a catalytic for possibly being clogged.
While neither of these tests are scientific, both can be effective to diagnose a bad catalytic.
Toasty....are you pulling out the chemistry set to test the exhaust? (j/k)