Tex. Tax Code § 162.009
To enforce this chapter, the comptroller or a peace officer may stop a motor vehicle that appears to be operating with or transporting motor fuel to examine the shipping document, cargo manifest, or invoices required to be carried, examine a license or copy of a license that may be required to be carried, take samples from the fuel supply or cargo tanks, and make any other investigation that could reasonably be made to determine whether the taxes have been paid or accounted for by a license holder or a person required to be licensed. The comptroller, a peace officer, an employee of the attorney general's office, an employee of the Texas Commission on Environmental Quality, or an employee of the Texas Department of Licensing and Regulation may take samples of motor fuel from a storage tank or container to:
Tex. Tax Code § 162.009