If you use a Carloop and establish a connection to the internet (or to the Particle Cloud, which requires internet), then you can easily identify the IP address of the Photon/Electron/RedbearDuo installed on the Carloop. You can easily do that on the Particle Console.
If the car has its own connection to the internet, then you would need some sort of sniffing device (WiFi, Cellular, Bluetooth or BLE to match what the car uses) to monitor the data packets to pick out the IP address. This is probably easiest away from the city where the only signal will be your car. In the city, you would probably get a huge list of devices on the internet and you would have no idea which device is your car and which devices are your neighbours'. I would guess that Bluetooth or BLE need at least intermediate experience in electronics and network communications; the WiFi would be advanced; the Cellular would be expert level.