The most reliable indicator is a measured change in the sound level differential between interior and exterior. At initial deployment, establish a baseline measurement: measure ambient noise on the open floor, then measure interior ambient with the pod door closed. Record the differential. Repeat this measurement quarterly as part of the scheduled maintenance review. A differential that has narrowed by more than 3 dB from the installation baseline — where the certified performance is 35 dB — indicates a potential acoustic integrity issue requiring professional assessment.