Only way to find out is to fly with them I reckon. Try to get one that you are comfortable with but who pushes you a bit. I've flown with instructors who I felt have gone too easy on me and haven't pointed out mistakes that I know I've made. On the other hand I know students who have dreaded going flying because of the hard time they were given by their instructor.
At the start I flew with five or six different instructors at my flying club, but now try and use just one or two that I feel most comfortable with.