We propose employing RIXS characterization to qualify the electronic structure of sub-stoichiometric WO3-x thin films, in view of applications such as transparent electric contacts. Oxygen-deficient WO3-x exhibits high electrical conductivity, concurrently it displays enhanced optical absorption within the visible-to-NIR energy range, which poses challenges for optimal applications. We aim to verify the hypothesis that this enhanced optical absorption arises from d-d excitations within the W 5d states, facilitated by the increased nominal electronic configurations of 5d1 and 5d2 as a result of self-doping. Once our research results verify the hypothesis, they will yield a comprehensive understanding of the underlying mechanisms driving the enhanced optical absorption, thereby providing insights to mitigate this effect, and contributing to advance the utilization of WO3-x as transparent electric contacts.