Some of the best advice I’ve read about testing Faraday cages is to put a strong light inside the cage and test inside a darkened room at night. Look for light leakage along seams, handle attachment points and the edge of the lid. Use metal tape to seal off the light leakage. The explanation was that the leaks would be small enough to prevent a phone’s or radio’s signal from getting through but big enough to allow EMP to enter. Since I do not have an EMP generator, I was not able to test this; however, it was easy enough to do.