Hi Christopher,
My first job post-undergrad was as a caseworker for one of the state's welfare offices. I actually purchased my first property while working there, and the skills I learned from interviewing clients and validating information proved extremely beneficial when evaluating tenants. One of the strategies I've developed is to create a standardized narrative that I've woven into my presentation while showing my apartment units. I don't ask any yes or no questions while talking to prospective tenants. Everything is left open-ended, and I listen a lot more than I talk. It's amazing what kind of information someone will tell you. Greg and Deanna mentioned some good indicators above. If you hear a lot of complaining about a previous landlord, that should prompt further investigation. If there is an immediate excuse for any deficiency in their application, such as a brief pay history, that should prompt further investigation.
Be just as wary about tenants that show passive-aggressive behavior. While that's slightly different from "knowing the system", the outcome can be just as miserable. The biggest mistake I've made so far is renting to a passive-aggressive tenant. How do you pick them out? Listen to how the prospective tenant talks about your place to whomever they've brought along. If you notice someone is overly critical about your unit, that should be a red flag. If someone requests a lot of accommodations or wants to change the lease at the last minute, those should be red flags as well. I had a tenant who made small remarks about the unit, such as a few paint specks on the floor from the fresh coat we had applied. No big deal, right? When I reviewed the lease with the tenant, she wanted several changes made to it. Afterwards, she started asking to store things in the foyer of the building prior to move-in.
Taken together as a whole, you'll start to recognize when someone's angling to take advantage of you. There are always really good actors out there, but doing a thorough screening, in addition to active listening throughout the process, will save you from a large majority of unsavory tenants.
Hope that helps!