There's an endpoint for getting information about a user - called /me. I can get a number of fields from it, including is_staff, which, presumably, indicates if a user is an SE staff member.
Could is_moderator also be added, where it returns true if the user is a moderator on the site specified? It's occasionally useful to do this, such as for applications where, for example, I'd like staff and diamond mods to have some privileges that non-mods don't get.
/me/associatedor/users/moderatorsendpoints.