I assume it's well enforced because so many of us have talked here at one time or another about being asked to show our AE cards and our BPs when entering the lounge. If you are E or SE, your card will get you in the lounge whether you have upgraded or not. The Prestige card alone won't get you in, nor will being up front with a cert or a flight pass upgrade. For those passengers who are on full J with no AE status, the receptionist can call up the record and see pretty quickly that it is J.