How long have they been teaching? What skill levels and ages, and when in their career?
Experience makes a vast difference; the longer a teacher has been teaching, the more likely they are to have seen and dealt with particular student problems. Teachers who are excellent with children or even teenagers may not be good with adults, and vice versa. Superb teachers of advanced students may not have a gift for teaching beginners the basics of the instrument. It’s generally preferable to have a teacher who takes beginners because they enjoy teaching beginners, rather than because they don’t have the skills to teach advanced students, but the latter may nonetheless provide excellent instruction to the beginner.
