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.

Just an observation - it would require a lot of management.  

I doubt the police would do anything based on your observation and could be a problem for you if you produce those pictures.  

MA, TN, MD, NJ all have different rules, but we all observe "landlord/tenant law" per se.Google "Nolo".

I let tenants list any observations like that, then I ask them what specifically if they something fixed.