There are many qualified instructors, but many more who are purposely deceitful. Unfortunately, Firearms, Martial Arts, and SCUBA are areas of expertise where it is common for posers and the simply ignorant to make their living. All three areas of study involve the risk of serious harm or death. Make sure you know who has trained your trainer. Simply dropping names like Ayoob, Murphy, Farnam, etc. is not good enough. Ask for details about your instructors experience. If they can't pick up their cell phone and text or call one of their instructors, they are probably being less than honest.
Read the great article linked to below (From Integrated Skills Group).