The knowledge of the oxidation state of the transition metal cations in Sr2FeMO6 (M = W, Mo, Re and Sb) double perovskites is of paramount importance to understand their appealing magnetoresistive or magnetic properties. We present a systematic investigation of the valences of Fe, W, Mo, Re and Sb cations in these perovskites using three different and complementary techniques of analysis. We have used a diffraction method, neutron powder diffraction (NPD), coupled with the bond-valence model; and two spectroscopy methods, X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). These two techniques are also complementary since XPS analyses the surface of the samples whereas XAS probes the bulk material. The analysis of the Fe K-edge spectra of the four samples shows a clear shift of the Fe K-edge as the valence of iron increases in the sequence M = W, Mo, Re and Sb. In addition, XANES pre-edge structures unveil a progressive reduction in the occupancy level of the Fe-3d band as the oxidation state of iron increases along the sequence M = W, Mo, Re and Sb. Finally, XANES computations have determined the electronic structures of Sr2FeWO6, Sr2FeMoO6, Sr2FeReO6 and Sr2FeSbO6.