When I was old enough to select my own instructors (as opposed to my parents, grandfather or friends choosing for me), I didn't really ask any specific questions, but rather did it the old fashioned way -- I looked the instructor in the eyes and went with my intuition. If I liked what I saw or felt, I joined. If I didn't like what I saw or felt, I didn't join. And as others have said, I did look at the students as a measure of the instructor's ability and also as potential surrogate instructors to whom I could also learn. But I gave much more weight to the instructor himself. Many great practitioners have students with poor ability. The one constant with all my instructors was that they all took the initiative by constantly doing research in an effort to continuously develop their abilities. They did not rest on their laurels but instead kept innovating, changing, growing and learning. Those who are interested in that path are easily and clearly distinguishable from those that are primarily interested in other things. You can see it in their eyes.