How to Test Your EMP Shielding or Faraday Cage

The easiest way to test your EMP shielding at home is to use a quality radio. Simply turn it on, tune it to a station with a strong, clear signal, then place it in the cage and seal it up. If you can still hear the station, your cage fails the test. But if it goes quiet or to static, it passes this simple at-home test.

[TheHillsHaveFPV]

this isn't a very good test, as you don't know how much the signal has been attenuated.
It might be enough to block a weak radio signal (the strongest radio signals are still relatively weak) but a few layers of tinfoil probably isn't enough to protect from a CME or EMP