In the martial arts, when a student shows a keen interest in the martial arts and is demonstrating excellent technical skills, the student is usually encouraged to become an instructor. Once in the instructor program, if the student keeps attending and paying, he or she will probably become an instructor. Therefore, there are many certified, but incompetent, instructors; and many more have no certification whatsoever.
Since there are no local, state, national, or international laws that regulate who may teach the martial arts, anyone can do it. There only way to know whether or not a person is a real instructor is in the quality of the instruction.
Quality instructor’s use a tell-show-do approach: they tell you how to do it, show you how to do, and then have you repeatedly practice doing it until you can do it. At a typical class at the school at which I teach, there are at least three doctors and two janitors, a teenage genius that knows everything and a downs syndrome man, the able bodied and one totally disabled veteran, and there are athletes and klutzes. They all leave class having learned something, having had a workout, having improved their Taekwondo skills, having more confidence in their abilities, and, most importantly, having had fun; and I am not near the quality instructor as is my instructor.
At every place I have ever taught, there have always been times when some black belt or student from another art or school would visit to check out things or give his or her opinion. It is to be expected, so most instructors do not mind. If they are confident in their art, their knowledge, and their abilities, then they enjoy a challenge.