Externally; Most U/C Crown Victorias have the black aluminum rims with chrome Ford centercaps. A lot of the times they will be white, black, or dark blue. They have black painted grilles, not the fancy color-keyed ones. Same goes with the mirrors. (This reduces prices a little bit due to less fancy-ness.). Many of them have antennas located on the trunk, and a spotlight on the driver's side windshield pillar. Not a lot that I've seen have the push bars on the front. On the trunk will be a black badge saying "Police Interceptor". Inside you'll probably see a partition for back seats. Black door handles... That's all I can think of right now. Of course, they are very identifyable.

Otherwise, to seperate them from the Crown Victoria LX's, they have heavy-duty cooling systems, dual exhaust, heavy-duty transmission, calibrated speedo's, and a few other things.