I'm going to go ahead and assert that they can't tell. A Faraday cage is just a deliberate construction of a situation that happens all the time anyway. Hospitals have lots of shielded rooms in and around the radiology department. The basement of a steel building is basically the same. So is anywhere on a ship. My aged house has lath and plaster walls that can simultaneously survive a nuclear blast and also block Wi-Fi unless the amp's turned up to 11. There's no sensor in an iPhone that could tell that it's in a specially-constructed Faraday cage instead of a plain old dresser drawer in my bedroom.