Like someone that would not likely walk off and leave you holding the bag?

I've had two occasions where the reference they listed told me not to rent to them because they were violent, late payers, or too needy.Not doing a criminal, credit, background check, which might have revealed criminal or credit problemsBeing slow to evict (mentioned by others)Believing their sob storiesAccepting partial paymentsFailing to document every conversation or interactionNot walking through the property to make sure they're taking care of it (I now walk through every 9 months so I can see problems in different seasons)We've found crack pipes and other paraphernalia after they moved outWe've discovered piles and piles of trash (some bagged, some not) in closets, basements, sheds when trash cans and pickup service are included in their rent - all they had to do was carry it out, at no cost to themWe've Found smokers, pets, extra peopleOverall, I've had great success as a property investor. 

Also am about completing my book on real estate but still don't want to let the cat out of the bag for now because massive work needs to be done to get it to the right people.

Tenants are a mixed bag and you should screen them well.

.  :/  It is a long story of how we figured it out, but essentially bags over the smoke detectors and drug paraphernalia in the trash gave the situation away. 

On your next day off want you to pack your bags for an overnight trip.  

Not having one is like walking into a networking event with a paper bag over your head!

A good way to classify A - B - C - D properties are by the shirts your tenants are wearing.A Properties = White collar professionals wanting nicer amenities to entertain friends.B Properties = Blue collar working class, bread and butter cash flow tenants that respect the property.C Properties = T-Shirts week-to-week paycheck residents, usually a mix of Section 8 and working classD Properties = NO SHIRTS gathered on the front steps drinking 40's out of a paper bag at high noon.

We supply anything that is shared (toilet paper for shared bathrooms, trash bags, dish soap, etc) so that tenants don't have to worry about whose turn it is to purchase supplies.

Left behind 48 bags of garbage.