RF Meters:

RF meters measure Radio Frequency, Microwave, and High-Frequency Electromagnetic fields.

Source:

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: Safe and Sound Pro ll Meter

Recommended device for advanced users: Gigahertz Solutions - HFE35C RF Meter Kit