For the purpose of ascertaining the correctness of any return or for the purpose of determining the amount of tax due from any person, the director of revenue or any employee of the director of revenue designated in writing by the director of revenue, may hold investigations and hearings concerning any matters covered by sections 144.010 to 144.510, and may examine any books, papers, records or memoranda bearing upon such sales by any such person and may require within the county where the person resides or does business the attendance of such person or any officer or employee of such person, or of any person having knowledge of such sales, and may take testimony and require proof for his information. In the conduct of any investigation or hearing, neither the director of revenue nor any employee thereof shall be bound by the technical rules of evidence and no informality in any proceeding, or in the manner of taking testimony, shall invalidate any order, decision, rule or regulation made or approved or confirmed by the director of revenue. The director of revenue or any employee thereof holding such investigation shall have power to administer oaths to such person or witnesses.