A good point that I wished to copy here:
Perhaps the best way is not to regard “teachers” as a general category, but as a (very) specific resource. The moment you do that, what you will be looking for in a teacher is his/her skill in helping your progress in a limited area, not a general panacea.
So, the best advice I can give to students in this moment in time is: Start from a specific problem. Then a good teacher will be one that helps you towards the solution of that problem. Then move on to the next problem. There are very few people in this world (maybe no one) that can solve all the problems of a student, and therefore it is unfair to criticise teachers on this basis alone.
Interestingly enough, because students rarely (if at all) follow the advice above – and most of the times are totally unaware of what problem they are trying to solve, being simply aware of a vague dissatisfaction with how things are presently – they end up being totally unable to judge the help they may be receiving. And sometimes help may come form the most unlikely of sources.
And so one gets all sorts of delusions about how to check if someone is a good teacher or not, when the real question is not if someone is a good teacher, but rather who can help me to solve my problem.
I have learned more about piano technique from persons primarily involved with the martial arts, than from any master class from internationally renowned concert pianists.
The point is, if you know your problem, anyone – even the taxi driver taking you to the airport – can be a good teacher if s/he offers you a way out of your predicament.
A really good teacher, of course, will be able to tell you not only how to solve your problem, but also what the problem is in the first place – although some bad teachers: the ones with strong theories about everything will try to explain your predicament in terms of their own logical system, even when it is not applicable.
Then human beings are fussy creatures. Having someone tell us what our problem is and supply a beautiful , elegant and feasible solution is never enough. There are egos to be protected! So a truly superlative teacher will do all that and in such a way as to protect our sense of self-esteem as well.