I don't think you'd use a gaussmeter but rather a mobile base station so you can measure the incoming pings

This was the approach used by SRLabs when testing their Blue Merle software for IMEI switching

https://github.com/srlabs/blue-merle?tab=readme-ov-file#imei-randomization 

 doesn't necessarily have to be mobile radio protocol, just something that someone can pick up, doesn't have to be strong, about 20 metres is probably enough, and that won't detect it