Specific to the Barnet Shale area, given that there was so much leasing activity over the past two years, if your potential seller has executed an Oil & Gas lease, then they at lease have part of the mineral estate. You can search the Tarrant County Deed records to find this.
The norm for the Fort Worth area was for the mineral estate NOT to have been severed at a earlier time, prior to development of the subdivision, and thus passed to the first homeowner, and each subsequent owner, unless one of them reserved part or all of the minerals when they sold. There are, however, many newer neighborhoods that have had all of the mineral estate severed from the surface estate, thus the surface owner has no mineral rights.
The only way to know for any tract, is to trace the title chain (read every deed and conveyance) from the current owner back to the original grant from the state.
Dial Boles, Broker, Landman