(a) The Secretary may investigate any complaint of any violation of this title. Upon his own initiative, the Secretary also may conduct any investigation he deems appropriate and advisable to develop information on prevailing procedures in commercial quantity determinations and on possible violations of the provisions of this title, and to promote accuracy in the determination and representation of quantity in commercial transactions.

(b) The Secretary shall weigh or measure and inspect any package or amount of a commodity possessed, offered, or exposed for sale, sold, or being delivered, to determine whether it contains the amount represented and whether it is possessed, offered, or exposed for sale, sold, or is being delivered in accordance with this title.

(c) In carrying out the provisions of this section, the Secretary may employ recognized sampling procedures to determine the compliance of a given lot of packages on the basis of the result obtained on a sample selected from and representative of the lot.

(d) For the purpose of this title, proof of the existence of a weight and measure in or about any building, enclosure, stand, or vehicle in which or from which it is shown that buying or selling is commonly carried on, shall be presumptive evidence of regular use of the weight and measure for commercial purposes and of use by the person in charge of the building, enclosure, stand, or vehicle.