There's no failsafe method short of using a police laser. But there are a few shortcuts, if you've got some luck.
Most big-city airports now have laser-based wind shear warning systems. They'll set off laser detectors a mile or more away. Many newer vehicles with sat nav systems will cause laser detectors to false uncontrollably when held anywhere in the vicinity, and not necessarily in front of the nav screen itself. Most new Audi, Infiniti and Nissan models with sat navs will cause this.
And Lexus LS430s with ACC (Adaptive Cruise Control) systems use a laser to measure following distance. They'll also cause uncontrollable falses.
Valentine Ones frequently false on laser in reaction to neon lights and to LED taillights in cars close ahead.