Testing Faraday Cages
Matt Blaze tested a variety of Faraday cages for phones, both commercial and homemade.
The bottom line:
A quick and likely reliable “go/no go test” can be done with an Apple AirTag and an iPhone: drop the AirTag in the bag under test, and see if the phone can locate it and activate its alarm (beware of caching in the FindMy app when doing this).
This test won’t tell you the exact attenuation level, of course, but it will tell you if the attenuation is sufficient for most practical purposes. It can also detect whether an otherwise good bag has been damaged and compromised.
At least in the frequency ranges I tested, two commercial Faraday pouches (the EDEC OffGrid and Mission Darkness Window pouches) yielded excellent performance sufficient to provide assurance of signal isolation under most real-world circumstances. None of the makeshift solutions consistently did nearly as well, although aluminum foil can, under ideal circumstances (that are difficult to replicate) sometimes provide comparable levels of attenuation.