There are a number of questionnaires, one of the most common is Beck's Depression Inventory. These questionnaires can be used multiple times over treatment to measure whether improvement is being made. You can find a number of different questionnaires even in self-help books, based on the symptoms of different forms of depression. Dr. David Burns also has a self-help book and questionnaire. Sometimes therapists have performed these questionnaires so many times, they know the symptoms by heart and only perform an interview. During the first interview or two, I allow patients to talk freely rather than try to structure the session.