A method for detecting a latent environmental effect (such as a corrosive environment under insulation) or a latent structural change (such as a crack in a concealed structural member) at a known remote concealed location. The method of the instant invention includes three steps. The first step is to use a harmonic electromagnetic transponder at the known remote concealed location of the latent environmental effect or the latent structural change, the harmonic electromagnetic transponder having a reactive portion which reacts to the latent environmental effect or latent structural change to modify the harmonic emission of the transponder. The second step is to remotely interrogate the transponder by directing electromagnetic radiation at the transponder. The third step is to use the harmonic emission of the transponder to remotely determine the latent environmental effect or the latent structural change.