One of the most common instruments an officer will use to catch you
speeding is a radar gun. In this case, radio waves are
transmitted at a specific frequency. The waves bounce off of your
car and are picked up by a receiver. The change in frequency of
the waves will tell the officer how fast you are going.
Laser guns can actually pinpoint a specific car more accurately than
radar. The laser gun can not be detected by a radar detector so
they are the preferred instrument in some cases.
Aerial detection using a helicopter or plane can time how long it takes
a car to travel on a stretch of highway to determine the speed.