There really isn't any way to test it. Just because it blocks your cell phone doesn't mean it will have any effect against an EMP.I bought a 1 gallon metal paint can from Lowe's to test as a faraday can. I put my cell phone in it and tried to call it from another line and it wouldn't go through...Is that the true test? only test?
've seen folks say that a microwave oven would make a good cage because it blocks their cell phone. I put a remote controlled toy in mine and could trigger it from anywhere in the house using the tiny, weak remote. It might block cell signals, but didn't block a wimpy radio remote. Even if it did, that still tells me nothing about whether it would block an EMP or not.