How to inspect your catalytic converters
Thanks to Doug Lewis :
It's pretty easy to check your cats for damage. You'll need to put the car up on jacks, remove the two 18mm bolts that hold the cat-back exhaust to the catalytic converter assembly. Then disconnect the oxygen sensors and remove the front one for ease. Then remove the four 15mm nuts that hold the catalytic converter assembly to the manifolds. Give a little tug and out she comes. Take a look down into the rear cat chamber and look for a white, shiny powder. It will look like beach sand. A rusty looking powder is OK. If you find the sand, better get a new set of cats. If not, then blow them out with compressed air and reinstall them. They will last a LONG time if the engine is kept in good tune and running right. About the only thing that will kill the cats are a stuck thermostat or a rich mixture for a long period of time.
If you do find "the sand" and are debating on whether or not to replace them, read What can kill a SHO engine? before you decide.