I believe that what you’re looking for is called a “spectrum analyzer”. You’ll need a calibrated antenna, too.

I’m guessing that SAR (specific absorption rate) is usually measured with a block of soggy flesh-imitating goo that’s set up as a calorimeter, though. You could look up how the FCC defines SAR, perhaps.

The consensus standard for this sort of test is IEEE 1528, and you are absolutely correct. The test uses a gel phantom (the standard provides a few formulations for different frequencies but most are a mixture of water, salt, sugar, and a thickener) and it is poked at with a sensitive probe to measure local temperature rise.

thanks to both of you for the specific info

(or RF radiation in general, wether related to wifi, gsm or other sources)

Wrong sort of radiation. A mobile phone emits radio waves not ionizing atomic particles. No ionizing radiation of any kind in fact.

Which is why mobile phones are completely harmless (from a radiation perspective anyway).

Careful not to go out in the sunshine though.