Showing: If the prospective tenants pass that stage, we invite them to see the property, at which time we observe and find out more about the prospective tenant.

Over that two years, I have observed they keep their aparments in bad condition, are very loud and bothersome.

And sites like padmapper and zilpy, and zilliow and Craigslist help to do that survey; I bookmark the search for those so that when the time to increase comes along, I click the bookmarks and observe the competition in the market. 

I've instructed the tenant to file a report if any illegal activity is observed and to take photos of the issue and send them to me.

Have you observed them walk; if so, how is their gait pattern? 

Also, for moral compass type questions an in-person interview would be best so both the verbal and non-verbal communication can be observed.  

Be prepared to observe:* Demeanor, interpersonal skills, communication style.

I can see, clearly, you have seen some red flags and are leaning towards not renting them.Furthermore, I sense, per my observation of this thread, a bit of anxiousness in renting out the unit quickly.

But honestly if you are happy with an expert that works in courts and legislature rather than observes in the real world, I have no reason to argue with you.

Its presence may exist without the knowledge of the structure owner and may be concealed from the untrained observer.