First and second thermal sensors measure the respective temperatures of portions of a surface of a structure such as an aircraft component. An alert signal is emitted if the temperatures of the surface portions are substantially different. An energy source causes heat flow within the structure. Subsurface irregularities such as disbanded areas between composite layers and foreign materials obstruct heat flow within the structure and cause proximate surface portions to exhibit different temperatures. A non-alert signal may be emitted if the temperatures of proximate surface portions are essentially the same.