This work provides techniques to separately determine temperature and duration of a thermal pre-load on a polymer matrix composite with a focus on long-term load (max. 400 days) without massive polymer degradation. The aim is a non-destructive, rapid, robust and precise in-service method to characterize incipient heat damage. A commercially available composite 8552/IM7 is investigated. Infrared spectroscopy of the surface and bulk material traces thermal degradation of the polymer. A multivariate (chemometric) data analysis was performed. The reliability of the calculated values for time (±19 d) and temperature (± 12°C) is increased by including other parameters with various degradation velocities such as mass loss of the composite and color changes and binder degradation of a typical top coat. The residual strength of a composite with unknown thermal history can be predicted and thermal loads such as heating in an oven and hot air are compared.