We show, for the first time, that the density of states of a magnetic thin film covered by an insulating barrier may be measured by using spin-resolved X-ray photoemission spectroscopy (SR-XPS). The basic idea is the lack of density of states of an insulator from the top of the valence band to the Fermi level. In this range of energy, the density of states of the underneath magnetic layer can be identified. This simple principle is demonstrated here for the [001] Fe/MgO system. From SR-XPS measurements, a polarization fully integrated in the κ space and corrected from remanence near 42% is obtained. Furthermore, we evidence a weak Fe-O hybridization and an enhancement of the Fe magnetic moment at the interface with MgO. These results are in agreement with previous ab initio calculations and certainly explain the fact that the Fe polarization is not destroyed in contact with MgO.