For the purpose of (1) determining whether any person subject to the requirements of this article is in compliance with any standard or requirement imposed pursuant to this article, or (2) investigating conditions relating to air pollution or possible air pollution, where the director is in possession of information sufficient to form a reasonable belief that air pollution or possible air pollution in violation of this article is occurring or about to occur, the director or his authorized representative, upon presentation of his credentials, shall have a right to enter into, upon, or through premises of persons subject to this article or premises whereon a violation of this article is reasonably believed to be occurring or is reasonably believed to be about to occur, to investigate, sample emissions, and inspect for compliance with the requirements imposed under this article or to determine whether such a violation or threatened violation exists; provided, however, that this Code section in no way limits the authority of any other person or entity to inspect any source or facility as may be provided by the federal act.