If you are close enough to see the blue lights between the grill or the Police LED sign on the parcel shelf then it's too late anyway!
Usually, they will be driving a high end four door saloon. Volvo T5 is common but the Skodas are starting to replace them. Dull colours are generally choosen as well. The car used varies from force to force and sometimes can be the unexpected (usually they got a deal to good to pass or a car maker has given them a model for PR, like Cosworths in the 90s and Scoobys / Evos in the 00s).
The arial and the 2nd rear view mirror use to be a give away but they are less common now. Some of the convert radio fits now are very impressive, arials around the rear window fitting for example.
They won't have dealer stickers on the plates as they make their own plates (with the exception of bikes) but again if you are close enough to read the small print on the plate you are too late, or you could just look up to see if the guy is wearing Police uniform and is waving at you.
In summary, there isn't any real way to be sure. Just keep your eyes peeled, check what the person is wearing as you get close enough, beware of fast closing 4 door saloons and cars doing 5-10 mph below the limit in lane 1.