A non-destructive method determines an amount of heat exposure to a resin-fiber composite substrate. A value of infrared energy reflected by a composite substrate is determined. The value of infrared energy reflected, or conversely absorbed, is correlated to a degree or amount of heat exposure. According to an aspect of the present invention, one method utilizes an infrared spectrometer to determine infrared absorbance of a composite substrate. The infrared energy of the reflected beam is then compared with the pre-determined value of infrared energy reflected off a reference heat damaged composite substrate to determine the amount of heat exposure.