Cat gone bad?

How can I tell if my catalitic converter is bad? I have an 89 C1500 and my current cat is at least 10 years old. I have been having some exhaust leak issues and am wondering how I could test if the cat is plugged. Any ideas?

Dave

you will know if your cat is bad if the following occur:
Loss of power.
Cat is glowing red (blocked or plugged)
excessive back-fires
In serve cases:
Unable to start or keep running.