According to RideLust, there are seven ways you can spot an unmarked police car. Some are subtle and some are obvious, but when combined can help you avoid that expensive speeding ticket. Also keep in mind that none of these tips are absolutes, since police departments also use confiscated cars in their fleets.
1) Learn what makes and models are used by police departments in your area. Unmarked police cars are likely to be the same make and model.
2) Look for multiple or unusual antennas, usually on the trunk or roof.
3) Look for strobe lights hidden in the grille, mirrors, windshield header or license plate frame.
4) Look for an absence of dealer advertising, such as stickers or license plate frames.
5) Check the wheels - police cars usually have steel wheels with hub caps.
6) Look for plain colors, like black, silver or white.
7) Check the license plate, since most states have special codes for city, county or state vehicles.