Buy this
It's a courtesy notification for a ticket if they are using the laser, but it will let you know if they are using radar or if there are cops moving on the road around you (rolling radar sets it off).
Learn where the cops sit usually. They have their "berry patches" were they will normally sit because they can write a lot of tickets without having to move much.
If you're not concerned with breaking laws, look into laser shifters and the proper way to use them.