Almost all have the Police etched or stamped into the blade. Since both the Police and Endura have been made with several grinds and at least two types of steel, the easiest way to tell any two apart is the handle shape, which hasn't changed. The Police will have a blunt tapered point on the butt end of the handle. The Endura lightweight has a squared off edge. The Endura SS has a slight taper, but it's still squared at some point.
There are also some limited runs of the Police without anything etched into the blade, and some with P.I.G. (Pride Integrity Guts) on them (for obvious reasons, this wasn't done for long).
The Police has undergone so many changes through the years that identifying it by any one feature besides the handle is extremely difficult.