You can check for a general obstruction. Feel the exhaust at idle, should feel like a bunch of little puffs. If it feels like a constant flow, then there is an obstruction in the system. What this does is stops exhaust from coming out, so the pulses will stop at the obstruction. Once the exhaust pressurizes enough to force its way past the obstruction, it comes out smooth. A clogged cat or a muffler with internal damage can cause this.
Another way, is a high RPM run. If it falls on its face earlier than it should this can indicate a failed cat. This can also indicate a lot of other problems such as a failing coil (I know you guys say it's more likely that a failed coil will show at idle, but I've had failing coils reveal themselves at higher RPMs, the coil breaks down internally and can't resaturate fast enough anymore), a clogged fuel filter or a failing fuel pump.
Any exhaust shop can check your cats, and will usually do it free. You don't have to worry about being lied to on this because it's against federal law to replace cats that aren't failed and these exhaust shops aren't willing to risk the 25K fine that such a violation will get them; if replacing the cat doesn't fix your problem you are liable to complain to the EPA about it and they take that stuff seriously.
The cats are covered under a separate warranty and can be as long as 10 years/100K miles. If you have the original documentation it'll show there.
