The health of thermoplastic induction welded joints was evaluated by monitoring the electromagnetic response to a time variant magnetic field through a local measurement of the attenuated and reflected field. For this purpose, a numerical model was developed to study how the joint interface affects this response in the presence and absence of a delamination and as a function of the electromagnetic characteristics of a susceptor. Using a single loop coil geometry as local magnetic field excitation and measurement, the frequency range at which the highest damage detection sensitivity occurs has been identified for different sizes of a delamination in the bond line. It has been found that the presence of a susceptor reduces the capability of damage detection for the considered values of electromagnetic properties. The obtained results are the starting point for further research activities aimed at developing an electromagnetically based sensitive layer for monitoring the health of thermoplastic joints, capable of detecting and quantifying joint damage and assessing residual fatigue life.