Solar fuel production through photoelectrochemical reduction of CO2 is a promising route to popularize the use of solar energy. However, the underlying mechanisms of these complex reactions are not yet fully resolved, hindering the rational design of novel photoelectrocatalysts. To shed light on this challenging problem, the X-ray photoelectron spectroscopy (XPS) and the near edge X-ray absorption fine structure (NEXAFS) of a number of molecular systems have been calculated using first-principles theory. First, it was found that both XPS and NEXAFS display specific features that correlate with the complex charge state and the coordination number of Ru atom. Furthermore, through the analysis of C 1s and N 1s XPS and NEXAFS spectra of key intermediates, we have identified clear fingerprints for metal-hydride and Ru-CO2 chemical bonding formation, two alternative pathways for catalytic CO, reduction. These results indicate that the understanding of the electrochemical properties of the electrocatalyst, as well as the reaction pathways, could be significantly advanced through operando X-ray spectroscopy experiments based on synchrotron radiation. We expect that these theoretical findings will be the basis of and motivate future experimental initiatives.