Among Non-Destructive Evaluation, vibration-based methods present some peculiarities. Structural damages change the dynamic behavior of the system, and vibrations should be used to detect these changes and to evaluate damages placed not accessible locations. These methods respond to the current tendency which is to integrate into the structure identication and monitoring systems; to this aim transducers and sensors are embedded with the mechanical system and driven by suitably designed electronic controllers. The major limit of this and the other indirect identication techniques based on frequency response measurements turns out to be the small sensitivity to local variations of mechanical characteristics: it reflects on a uncertainty on the identied parameters, since it implies a small curvature of any reasonable identication functional build on these quantities. An enlightening example for the application of the proposed technique is given through a beam-like structure coupled to a network of piezoelectric patches.