Get an exhaust back pressure gauge. Above-normal back pressure is a pretty decisive sign of a clogged cat.

Drivability-wise, acceleration will drop off significantly, along with fuel economy. Crankcase pressure will increase. The converter itself can also overheat, although that isn't necessarily a sign of a clog (rich mixture can do that too).

When one of the cats plugged up on my V8, the difference was audible -- the exhaust sounded constricted during acceleration.