Most of the management standards call for internal auditing to be carried out at least once a year by competent persons, correctly trained. This process, if carried out correctly, will be of great benefit to the company, providing detailed information about the parts of the company that work (and the parts that don't). The results when analysed allow for continual improvement and increased customer satisfaction.
The main problem today is that some organisations will try to audit the processes without knowing exactly how this should be done. The results are inconclusive at best and misleading at worst.
The only way to be sure and to get the best results is to have your auditors trained properly. Proper courses are based on the Standard ISO19011, Auditing Requirements for Quality & Environmental Audors and are usually two days in duration and successful candidates receive a certificate of successful completion.
The first day concentrates on the two main standards 9001 & 14001 and how these are applied to organisations. An end of day test confirms understanding.
The second day looks at audit practices, techniques, routines and form filling, and finishes with tests of understanding and competence.