There are two MAJOR methods in XPS - method of internal reference standard and method of external reference standard. Most effective to determine an oxidation state of transition metal is the second method. You have to compare Your XPS spectrum of the sample under study with the known spectrum of known and stable oxides for that transition metal, which oxidation state You wish to determine. Namely, You have to compare the TM 2p spectra with that for reference oxide samples.

Also O 1s spectra have to be compared. For detailed procedure please have a look our published papers - we did nearly the same with the unknown material in order to characterize its compositions. Thank You.

Here is described more detailed procedure in the case of unknown mixed valence states of Copper. Also OVERLAY method is shown.

Valence states of copper ions and electronic structure of LiCu2O2

Zatsepin, DA, Galakhov, VR, Korotin, MA, et al.

Here is much more complicated case: implantation of Si into Si waffer

XPS analysis and valence band structure of a low-dimensional SiO2/Si system after Si+ ion implantation

Zatsepin D. A, Mack A, Wright A. E, et al.

All papers are available from my page for free. If You have some additional question, please don't hesitate to write me a letter. :)

P.S. The same it is possible to do with x-ray-emission spectra of Oxygen.

Here it is also an example:

Soft-x-ray-emission study of the influence of Li1-doping, irradiation, and plastic deformation on CuO

Dmitry A. Zatsepin, V. R. Galakhov, B. A. Gizhevskii, E. Z. Kurmaev, V. V. Fedorenko, A. A. Samokhvalov, S. V. Naumov, R. Berger