gps trackers work in two different ways. Either they actively track and transmit your location (can jam the radio transmission, can fake out the gps.. but it also screws up your gps on your phone), or they track and record.. with the device recovered later. The only way to detect this is to physically inspect your car. You can fool or spoof the gps coordinates with cheap gear built from stuff bought at radioshak, but that's above my technical expertise.