Here in Vegas they do a visual, and dyno test. The vehicle goes on the rollers, probe in the pipe, and they'll mildly get it up to 45mph or so. As long as you aren't blowing clouds of smoke, it'll pass the sniffer test. When it comes to the visual, most smog techs have no idea what they're looking at. They will check for a cat, fuel cap, and glance under the hood. I had my nitrous system installed, and still passed. lol