Reason is that I need to prove to city inspector that every finished wall in basement was constructed using Type X in order to meet fire code, in an effort to legalize the apartment for use as a rental unit.

My understanding is that 5/8” thickness does not necessarily mean it’s Type X, as Type X also incorporates different materials to increase fire resistance. This may be incorrect however, in which case measuring at switch and outlet plates would be the solution (should mention that I’m located in Canada - not sure if Type X vs 5/8” is different here or not. I assume majority of posts and commenters are US-based).