The board of directors of any employers' mutual liability insurance association shall be entitled to inspect the plant, workroom, shop, farm, or premises of any subscriber, and, for such purpose, may appoint inspectors, who shall have free access to all such premises during the regular working hours. The board of directors shall likewise, from time to time, be entitled to examine, by their auditor or other agent, the books, records, and payrolls of any subscriber for the purposes of determining the amount of premium chargeable to such subscriber.