Cal. Rev. & Tax. Code § 60606

The board or its authorized representative may examine the books, records, and equipment of any interstate user, supplier, exempt bus operator, government entity, ultimate vendor, qualified highway vehicle operator, highway vehicle operator/fueler, train operator, pipeline operator, vessel operator, or person dealing in, removing, transporting, or storing diesel fuel and may investigate the character of the disposition that the interstate user, supplier, exempt bus operator, government entity, ultimate vendor, qualified highway vehicle operator, highway vehicle operator/fueler, train operator, pipeline operator, vessel operator, or person makes of the diesel fuel in order to ascertain whether all taxes due under this part are being properly reported and paid.