How do I know if an Android tablet supports Project Butter?
"Project Butter" is Google's name for the UI latency enhancements implemented in Android 4.1 Jelly Bean. Any Android device running Android 4.1 or later will have those enhancements.

Remember, though, that software is only part of what makes an interface smooth and 4.1 isn't a cure-all for lag. Slow hardware will result in a slow UI regardless of the OS. Your best bet is to find a tablet with 1) 4.1 or higher, 2) good specs, and 3) good reviews of its performance.