Most people I talk to about police issues have no idea as to the name, rank and assignment of the person they were dealing with. This is unfortunate, since this information is readily available just by looking at regularly uniformed officers. Here's a primer.
There are two main ways to identify police personnel: through the uniform and through the vehicle. Obviously this won't help you identify non-standard personnel - bike cops, undercover/plainclothes people, etc - but it'll help you with most.
Uniform: All regularly uniformed officers wear a hat with their badge number on the front. These badge #s are unique. If you know the badge # you know the officer. The badge # is set in a design that identifies the rank of the person. The badge (with number and rank design) and name also appear on the left breast of the uniform. The collar of the uniform typically carries a pin identifying the officer's assignment. All officers Sgt. and below wear a blue shirt. Everyone above gets a white shirt.
Vehicle: All regularly marked vehicles identify the vehicle's assignment and the vehicle # with a 3, 4 or 5 digit code. This code appears on the bumper, trunk and front fender of the car. The first few digits are typically numbers - this is the number of the area (typically a District) to which the vehicle is assigned. The next few numbers are the car's unique ID. For example, car 263 is 26th District, car three. The most common non district markers are T (Traffic), X or H (Highway Patrol) and A (Accident or Airport). If you know the car code and approx. time, date and location, you can locate the officer.
In Philly, cruisers are in the process of being shifted from Crown Vics to Impalas. Traffic units often use Explorers, while Highway Patrol has some Chargers. There are also a few other marked vehicles, usually Expedition type SUVs, for crime scene investigation, SWAT units and the like. The PPD also uses a number of unmarked units. Some of these are Impalas/Vics, but they also have a number of smaller sedans for detective and civil affairs units. These are typically outfitted with blue/red windshield lights and sirens. I have also seen a small Saturn that looked like an undercover/investigative unit, as well as late-model American vans (be suspicious of any van w/ tinted windows). Of course, they will also use seized vehicles, typically only for a brief period - Narcotics especially. Not much you can do with these except noting the TDL.