One thing that is always a clear mark and true of every real teacher is humility.
The reason is a very simple one. It is not a moral reason.
It is because the greater a person is, the greater he feels his insignificance and his lack.
By definition, a great teacher will have great humility.
Another mark of a real teacher is the feeling of responsibility.
It is essential for a teacher and is also a mark of the personality.
Sometimes you can look at two teachers and you don't see the difference.
Outside they may look very much alike.
One of the ways to find out is when you look at their disciples, and then you see what was in the teacher.
It appears in the disciples in a far more marked way and so it is a good way of assessing the kind of person the teacher is.
–Rabbi Adin Steinsaltz
From "The Open Gate", an interview with Rabbi Adin Steinsaltz in Parabola