Well, after I get pulled over and they walk up to the window, they usually go:

"Do you know why I pulled you over?"
"No officer, did I commit a traffic violation?"
"No, I noticed you have a illegal modified exhaust"
At this point I ask them politely if I can open my glovebox, and I reach in and take out the sound test papers that GReddy provided me proving that in CA, the exhaust is 100% legal. Looks very official and, while sometimes they call for their local "Street Racing expert", it usually deflates them quite a bit.
I also offer to pop the hood and show them that all my carb numbers are intact and valid, and I also hand him the EPA greencard for my cat. Usually this is enough information and 50% of the time, they don't bother popping my hood. If they call the "Street Racing expert", then he usually comes, and uses my car as an example of what's legal and instructs the other cop on how to spot illegal modifications. This usually makes the first cop feel dumb and start to crack really stupid jokes to make it seem like this whole pulling-over thing was just a friendly little checkup. They check for warrants, and I'm on my way, about 15-20 minutes late...