How to find a good instructor..
This is not meant to be the only list of questions you should ask your potential instructor, but I think it is a pretty good starting point.
1. Ask the instructor for a list of references. Specifically, people he has taught in the past. Ask them about their diving background, and what they thought of the class, and how they have put those skills to use.
What you should be listening for are people that have continued their diving and have found the skills they learned to be valuable to them. Personally, I am less impressed with an instructor that has certified hundreds of divers but only a small margin gained anything useful than I am with an instructor that only produces 5-10 divers a year, but all of them learned useful skills, engaged in diving, and doing quite a bit.
2. Ask the instructor what kind of diving he does when he's not getting paid. When was the last time he went diving for fun?
In my opinion, your instructor should be passionate about diving. I believe that someone that is truly passionate will do their best and work harder than someone that is jaded, burned out, and only goes diving when he is being paid.
I also think it is good for an instructor to actively play "one level above" what they teach, but I know several instructors that I think very highly of that do dive for fun, but do not regularly dive a level above what they teach. So do not let this be a single deciding point, just consider it a bonus.
3. Ask the instructor about his teaching style, and what the schedule for each day look like.
What you need to listen for is a teaching style that fits with your personality. Even great instructors may run their courses in a schedule/format that is incompatible with you; some may go for a "boot camp" mentality where the first dive of the day begins shortly after 6AM, and the day ends at 7PM. Others may not want to get started until 10AM and finish by 4PM. And everywhere in between.
If you're the kind of person that likes to start at 10AM then the 6AM course may not be for you, and if you're the kind of person that expects/wants a boot camp, the 10AM course may not be for you.
4. Ask the instructor about gear configuration, what configuration he dives, and what he would expect from you.
Some instructors are dogmatic and rigid on their gear choices, others believe in "doing what works". And some are once again in between.
For instance, while I am not dogmatic about the gear my students use as long as it is safe, because the majority of my experience is in the "hogarthian" style, I may not be able to offer very helpful suggestions to someone diving side-mount.
5. Ask the instructor what his expected outcomes for the course are. What does he expect you to be able to do by the end of the course. What does "success" mean. What constitutes failure.
Your instructor should have an idea of what he expects from someone, and he should be able to articulate that coherently and intelligibly.
6. Ask the instructor who his mentors were. How did he learn to teach the material he is presenting? Who did he "study under". How long did it take him to become an instructor for the course you are taking. What is his diving experience for the course you are interested in taking.
Depending on the course, your instructor should have mentored under multiple different instructors over a period of time. It took me a year to become a cave instructor once I really decided to complete the process, and it took me six months (and close to 100 hypoxic trimix dives) to become a trimix instructor. I probably took longer than others, but if your instructor was able to become a cave instructor in a month (or less), or a trimix instructor in a week, well how much experience does he/she really have in the subject?
7. Ask your instructor why he/she wants to teach a particular course.
There are many reasons why someone teaches, and the majority of them are valid. This may give you a better insight into your instructor.
8. Ask your instructor when was the last time he took a course, co-taught with someone else, or participated in a workshop.
No one knows everything. Someone that still co-teaches, takes courses, or participates in workshops realizes there is still more to learn and by constantly seeking new sources of information, they are trying to improve.
I hope this helps.
Ken