I took the case, shoved my CharlieCard into one of the card holder slots, and then tapped against the reader on a T ticket vending machine. No read, as long as the card was entirely within the case.
If it was a bit out of the case (more than a couple of millimeters) there was enough of the antenna out to confuse the machine. It gave me the "bad read, see attendant" error. If I had it out more than about 5mm, the reader recognized the card.
End on reads failed, which is also good.
Until I have an RFID reader handy--which is something I want anyway--this is the best test I can think of offhand.