There are TONS of threads like this on BP and are definitely worth reading.
Look into the Section 8 demand in your local market. To determine this speak with other investors and also get a copy of your local Section 8 Housing/Apartment List to determine supply and demand. Heck, the local housing authority can be a good source to gauge demand (the local Section 8 inspector is a great source and knows the demand well).
Personally, in my somewhat limited Section 8 experience, I've realized that I've had less headaches renting out 1 bedroom units to Section 8 tenants and more headaches when renting out larger units to Section 8 tenants.
There are exceptions to every rule and all landlords have different experiences but this is my experience.
Aside from that, there's the obvious - background check, credit check for eviction records/judgments, etc.
@Donald Stevens Jr. - I wrote a blog on the subject that has lots of advice.