If it's in town they'll have gotten your reading from a certain distance off, and then they'll stand in the road and wave you to pull over when you reach them.
In some cases they have more cops waiting up ahead that do the pulling over (used to do it this way anyway, not sure if they still do) after the radar cops let them know.
On the highway however, they'll sit inside the car, using it out the window (or using the one mounted to the side of the car), and then they'll chase you and pull you over.