1. A method of detecting the presence of a first metal inserted into a second metal comprising the steps of: measuring a second metal baseline object using resonant ultrasound spectroscopy; measuring an unknown object having similar dimensions as the baseline object using resonant ultrasound spectroscopy; comparing the baseline object and unknown object resonant ultrasonic spectra to determine if the unknown object has essentially the same resonance structure as the baseline object; and detecting the presence of the first metal in the unknown object by observing differences in the resonant ultrasound spectra in the baseline object and the unknown object.