Pitching coaches are charged with two main tasks: to improve pitch quality and to minimize risk of injury. Unfortunately, most coaches can do either or neither but not both. The demand for pitching coaches is far greater than the supply of good pitching coaches. The obvious result is a large number of pitchers learning from coaches who are simply not good.
The primary deficiency of most pitching coaches is education. There are countless pitching coaches in youth and amateur baseball who are hired simply because they played professional or semi-professional baseball. This undoubtedly looks good on a resume and is great for marketing, but it says a lot more about the coach's ability to play the game than it does about his knowledge or ability to teach it to someone else.
Other organizations will hand out what amounts to a step-by-step guide to pitching mechanics. At least one prominent national organization has a guide based on scientific research, and they make the research available for the coaches to read if they so choose. Unfortunately, even if a coach chooses to read the material, there is no guarantee that he will understand it. The coach's interpretation further obscures the science behind the guidelines which are someone else's interpretation of the research. This model is better than most, but is clearly not ideal.
In a perfect pitching world, all pitching coaches would be well versed in biomechanics - a sports science that combines mechanical physics and human physiology.
Throwing a pitch is a pretty simple task - accelerate the baseball toward the catcher's mitt - and the human body is the incredibly complex machine that executes it. A large collection of levers (bones) and pullies (muscles and tendons) create a kinetic chain from the pitcher's feet to his finger tips.
Newton's laws of motion describe the movement and acceleration, and human physiology determines how the levers and pullies of the body work together. Used in tandem, they can help identify mechanical inefficiencies and injury risk factors.
Pitching coaches that gloss over these subjects or pay no attention to them are cheating their clients and potentially endangering them. There are prominent coaches who teach things that are unquestionably inefficient and others who teach things that are physiologically reckless.
Coaches who are dedicated to their own continued education give their clients the best chance for success.
If you are serious about hiring a pitching coach, you should have some idea of what he will be teaching. You must educate yourself.
In the absence of knowledge, it becomes a question of trust.