Alix has hit the nail on the head.  I have prepared mineral title over 32 years covering 27 states for coal, oil, and gas companies.  If your client insists on verification of mineral ownership that is only the beginning of your problems.  the question goes beyond the typical answers you are searching for on a surface title search, i.e. who owns the mineral estate(s), who owns the operations rights, royalty interests, subordinations, etc....  You can not really scratch the surface of how deep a hole you are digging yourself into unless you really understand the legal, financial, and extremely time consuming task a mineral abstractor undertakes.  Several posts mentioned checking on the assessors office for who is paying the taxes, well paying taxes does not ensure ownership by any means of the imagination.  If you can't disclaim the mineral part of the search best advice is to not accept the assignment.