A system and method for inspecting a composite structure, such as to assess thermal degradation or resin curing, are provided in which the dielectric constant of the composite structure is determined using a microwave inverse scattering technique. The dielectric constant of the composite structure may be compared to the dielectric constant of one or more sample structures to determine the presence of thermal degradation or improper curing in the structure. In this regard, a system for inspecting a composite structure comprises a transmitter, a receiver, and a controller. The transmitter may be capable transmitting microwave energy directed toward the structure. The receiver may be capable of receiving microwave energy scattered from the structure. The controller may be capable of determining a dielectric constant of the structure using an inverse scattering algorithm and comparing the dielectric constant of the structure to a dielectric constant of at least one sample structure.