Best gps tracker detector?

There is no phone app that reliably detects a GPS tracker, because the hardware you are hunting does not transmit anything a phone can see — it silently receives satellite signals and reports position over cellular or Bluetooth — so detection is either a physical sweep or the operating system's own Bluetooth-tag alerts. Anyone selling an app as a "GPS tracker detector" is selling a placebo; a GPS receiver is a listener, not a beacon.

What actually works, in order of yield:

1. Physical sweep of the vehicle. Trackers are battery-limited, so they sit where they can be reached: under the seats and carpet, in the boot trim, behind the glovebox, in the OBD-II port under the dash, along the wheel arches, or in a magnetic box stuck to the frame. A torch and ten minutes finds more than any software.
2. Bluetooth scanning. A Bluetooth tag announces itself. Free scanners such as nRF Connect or LightBlue list nearby devices with signal strength; walk the car and watch which one strengthens as you approach a hiding spot.
3. The built-in alerts. iPhone shows Item Detecting You for AirTags and Find My-compatible tags travelling with you; Android has unknown tracker alerts under Settings → Safety & emergency, and Apple publishes Tracker Detect for Android. These are free, first-party, and the only automated detection that genuinely works.
4. The phone itself. Check for location sharing you did not set up (Google Maps → Location sharing, Find My → People) and for management profiles under VPN & Device Management on iOS or Device admin apps on Android.

Where Ultimate Phone Spy sits honestly: it is a parental-control dashboard for Android 4.0+, Windows and Mac, not a detector — it shows location history for a device you administer (location tracker). If you need to find a tracker, an app purchase will not do it. It does not cover iPhone.

The legal line matters here: secretly tracking an adult's car is stalking or harassment in most jurisdictions, and if you believe someone is tracking you, report it to police rather than pulling the device out yourself — it is evidence. Monitoring a minor's phone you own and administer is the lawful use case, and it needs to be disclosed.