Spy devices on cars are typically designed to track where the vehicles go, although listening devices can be planted as well. Determining if a spy device is installed in a car is fairly easy, so long as the device is not a passive one that is well hidden. Once uncovered, the devices are usually easy to disable or disrupt, although special caution and consideration should be taken if the device was installed by an employer.
Determine if a live GPS unit is installed. Live GPS units allow someone to track your vehicle in real time. They will be able to follow the vehicle on a computer map, tracing the car's route, where it stops and how fast it is going. Such devices are most common in rental cars and commercial trucks.
You can usually determine if you have a live GPS unit by searching for a stubby antenna installed on the roof of the vehicle. Such antennae are usually reserved for satellite radio or GPS. If disabling the unit is desired, simply cover the antenna with aluminum foil and wrap packing tape around it. This should hold up while driving and disrupt communications enough to prevent the unit from supplying proper coordinates.
Search for passive trackers. Passive trackers are placed on a car to record information surreptitiously. They are later retrieved and the data is downloaded, revealing a record of the vehicle's movement. Passive trackers usually work best when exposed at least partly to the sky. Look on the front or back dash for a small box. They may also be in the glove box, under the front or back bumpers, or elsewhere on the vehicle. The tracker may have a USB dongle for downloading data to computers and a magnet for attaching to the car's chassis. The box is usually weatherproof so it can be mounted outside. Reference a current spy gear catalog for assistance in identification.
Search for radio units. Radio beacons can be found with frequency sweepers, used to detect nearby transmissions. Beacons emit signals, allowing the car to be followed. Listening devices will also operate on radio frequencies. Signal sniffers or bug detectors are frequency sweepers designed to indicate the presence of spy devices. Lower-priced units will simply detect the presence of bugs, while higher-priced models will hone in on the location of hidden devices.