If you think that their activities go beyond recreational use (large quantities in the house, visible scales or bags, people coming and going in excess) it's time for a more serious conversation. 

This is my most effective tool un my tool bag.

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. 

Stop hanging out with your deadbeat buddies that spend all day drinking, talking sports, and otherwise wasting away.

My son has the genes but never had any interest in sports. 

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