How can you tell if your exhaust is bad?

My car's got this funny smell, and it sounds like crap. I can't describe the smell, but it's not a burning or exhaust kind of smell. It smells like a chemical. I only smell it with the engine runnning, in gear (auto tranny), and I'm stopped (red light, traffic, all that crap). It's much stronger when the A/C is on normal, not max. So, that would tell me it's coming from outside (since max A/C recirculates air). It's not that strong with the heater or normal vent on. As far as the sound goes, it sounds like a crappy CRAPPY old corvette (rotting, not taken care of). That uneven, dull, kinda bubly sound, at idle. I can't describe it well at all. I already know my muffler is bad. It's a magnaflow. The inner pipe is intact, but I can pull the outer shell off the car if I wanted too. It's completely detatched from the pipe. It makes a horrible rattle, on top of this other dull bubly mess. I'm gonna change my muffler, but I'm wondering if my cats are bad also. How can you detect something like that? There are no holes in them. It almost seems like my engine cant breeth. I have the ebay CAI which has been in place for 2 years, and I don't think that has anything to do with it. I can also hear my valves getting pretty loud when I try to merge on the highway, which is new. Also, if I'm on the highway (doing let's say 60), and I give it a little more gas, the exhaust tone gets deeper, and quieter. Rpms will stay the same. And of course if I give it more gas, I hear the valves. Oh, keep in mind, I'm a complete moron when it comes to exhausts. I did a search, and read all the posts, but I still need help.

Oh, if this helps any. It's a 94 6-banger. For the past year, the ehaust has been getting quieter and quieter at idle. And the car has been getting more sluggish too.

The cell is the check ingine light I hope? Nah, that thing never comes on while the engine is running. If I turn the key to acc but don't start the car, it will come on. Once the engine is started, it goes off.

CEL=check engine light. I'm a genious.

Well I went to Mineky (however you spell it). The guy put it on a lift, tapped on one of my cats with a screwdriver, and told me all 4 are bad. He also said I'd have to chenge my o2 sensors. Imagine that? The guy knows that much from a tap. Wow. Then they recharged my A?C, and told it would be 60 cash. I didn't have cash, so they said they trusted me to go to an atm, then come back. I never went back. SUCKERS. I'm glad too, cause my A/C blows warmer now than it did.

I'll probly get a bolt on system, and new o2 sensors, or the mil eliminators online, then find the cheapest place to do the install. Bolt on, i wonder is I can do it myself. I thought there were 4 o2 sensors.