As a landlord, I am constantly evaluating prospective tenants over the phone and in person.

One of the most important things to consider is consistency.  To stay out of trouble with discrimination, make sure that you ask the same questions to every prospect.  I would even have a checklist or point system that you use, including what kind of car they drive, what the exterior of the place that they currently live in looks like, and how many times they have moved in the past 2-3 years.  It even is a good idea to call back or mail a denial response.

I have also found that asking 'why are you moving from your current housing?' will prompt some interesting answers.  I even go as far as asking more than once, as the story tends to change after a couple of times.

More in a while, my time is limited this evening.

Jeff