Measuring the anomalous Nernst effect of magnetically ordered thin films on substrates is often hampered by the mismatch between different thermal conductivities and the unknown temperature gradient in the film. Here, we demonstrate that for ferromagnetic films deposited on a semiconducting substrate, the Nernst effect of the semiconductor can serve as an internal reference for estimating the local temperatures present in the film and substrate. The Nernst effect in semiconductors is particularly large in the temperature region above 200 K where the semiconductor shows a strong change in the charge carrier concentration. As an example, we have studied the anomalous Nernst effect of 28 nm thick ferromagnetic Mn5Ge3C0.8 films deposited on p-type Ge(111) substrates with the temperature gradient oriented either parallel or perpendicular to the film plane. From the additional contribution of the Nernst effect arising from the substrate and comparison with theory, we estimate the temperature gradient in the substrate and thin film for both configurations and determine the thermoelectric coefficients of the film.