RF Meters

Those meters will measure RF, Microwave, High-frequency electromagnetic fields EMF

Sources:

Cell towers, smart meters, Wi-Fi (e.g. routers and modems), Bluetooth devices (e.g. printers, headsets, wireless keyboards), cellular & cordless phones, remote controls (e.g. for your garage or car), microwave ovens, and any other device which transmits wirelessly.

Recommended device for beginners: Triple-Axis HF-B3G RF meter 

Recommended device for advances users: Triple-Axis HF-B8G RF meter