“Wouldn’t you say the cage fails if it doesn’t pass the ‘radio’ test? Could you please prove an alternative test.”
That would be true.
To be sure, you’d need multiple tests and many frequencies that could measure across an up to 80dB dynamic range. Not an easy thing to accomplish. Google “MIL-STD-188-125-1” for a lot of detailed information on a test apparatus.
The dynamic range is a key factor.
Then I would use a radio test multiple frequencies on the FM, AM and SW band and then use a couple of cellphones to the GSM and UMTS bands. You could even use a small TV and test UHF and VHF bands. I would say that after such a test you could be fairly sure you were covered.
How about just using a multifrequency scanner?