For the purpose of nondestructive testing (NDT), guided waves can be transmitted into a structure, and any defects or anomalies in the waves’ path modify the measured waves. Signal processing methods can be used to extract information about these features. In this work, an NDT method is demonstrated based on laboratory experiments for the case of a flat, rectangular, aluminum plate, which has a stiffener mounted underneath along the middle axis, such that the stiffener cannot be seen from the upper “outside” surface. Piezoelectric transducers are set up in a pitch-catch arrangement on this surface with the assumption that the location of the stiffener is unknown. When guided waves are induced in the plate by one of the transducers, the waves that are received by the other carry the information of the stiffener, as well as any defects in or boundaries of the structure. By transmitting from different points on a grid on the plate, the location and size of any geometry or material discontinuities can be identified. Hence, the developed algorithm reverse engineers the plate by mapping its edges and identifying the region of the stiffener.