I had a gun that was part of my equipment that was in bus accident. We got T-boned by a driver who ran a light. In any event, I thought the thing was damaged slightly so I found one of the signs that police use that tells on-coming traffic how fast there going, then I set myself in a lawn chair next to it and compared my readings with that of the illuminated MPH that flashed on the display. In the final analysis I found my pointer was about a mile per hour or so off from the street display.
However, I want to suggest being careful while using this method. You don't want to distract drivers by giving them the impression that you're recording the speed at which they're driving. So, use your friend's car, your family car, or someone that you know.