As you get information through interviews, you should be identifying both your positive and negative hallmarks that highlight your high performers and low performers. Having a short list for each would be ideal move to do so. Most important thing is to pay attention to the common themes that are most re-echoing. These themes will be significantly similar concerning the people with the wrong attitudes and why these attitudes are wrong, and also with the great attitudes and why these attitudes are great.
A simple hash marks system will be helpful to record these repeated themes and you will obtain the most dominant, positive and negative, characteristics for your organizational culture. Preparing a table that shows your three best and three worst employees representing positive and negative characteristics that are proved by behavioral examples over the past three years will be very efficient to discover your hallmarks.