Just my observation and preference.    

I found out that one of the tenants (family with one kid and another on the way) earns more than I do at my job (they probably make between around $45k between the two of them), and in the other unit, it's a couple with 3 kids and a dog, where the mom works full time (in a department store) and the dad goes to community college and gets stoned at home during the day (but I have no evidence of that other than personal observation when I repair stuff over there).The point is, as each week passes and I lose money paying for the loan on this property, I feel PWND.  

Just a few observations from someone who did this repeatedly over a 24 year active duty career:First, owner-occupied financing helps a lot, but as others have said above, the numbers still need to work.

Your observations are well taken.

Your typical big dog is going to bark less and pee on the furniture less but that's more personal observation than fact. 

Your powers of observation are legitimate enough, in my opinion.If your friend saw mail in a way that wasn't illegal, he could be a witness.  

@Dean Weltman I observed the same thing, those from CA were bashing the NY Post and one suggested the NY Times would be a better source.

I didn't monitor any of my tenants until/if I got a complaint from another tenant or I just happened to observe or hear some behavior that I needed to give them a warning about.So, in my case, since I lived in the same building, if they were throwing a loud party, I'd usually see or hear it.  

Now I have to pay a Marshal to observe them.

As a disclaimer though, that is just an observation, you should never just run a credit check on your applicants.