The health state of a structure depends on both the degradation of its mechanical properties during its life cycle (ageing) and to the presence of localised defects such as corrosion, cracks, or delaminations. We already described1-3 a method enabling the recovery of both kinds of information, and based upon a piezoelectric disk attached to the plate-like composite structure. The element dimensions are chosen in order to uncouple the frequency ranges of its thickness and radial vibration modes. The thickness mode is then exploited to monitor the homogeneous ageing of the structure through electrical impedance measurement. Whereas the radial vibrations, are used to launch and detect Lamb waves, which are known to propagate over long distances. The present work focuses more particularly on the reliability of the technique. The definition of a very simple "damage index" computed from the experimental data is proposed and its high sensitivity is discussed. A finite element model of real and hence complex damages is used in order to understand the interaction of the selected Lamb waves with the defects. A good agreement between the experimental observations and the simulations is found. The sensitivity of a given guided mode to a given type of damage can then be understood and then a predictive implementation of the experimental device can be envisaged.