Disclosed herein is a method of measuring a gap between exterior structures and interior structures. The method comprises directing a transmitted m-wave signal from an exterior surface of the exterior structure into the exterior structure and the interior structure. The transmitted m-wave signal is generated by a gap sensing device that comprises an electromagnetic dual-tuned resonant coil sensor. The method also comprises measuring a received m-wave signal with the gap sensing device. The received m-wave signal comprises the transmitted m-wave signal influenced by the assembly. The method further comprises determining a size of the gap between the exterior structure and the interior structure based at least partially on at least one measured characteristic of the received m-wave signal.