The yard was well tended, and there were no violations listed with the city for lawn or snow issues (they issues citations for lawns over 10" or sidewalks not cleared in 24 hours).  

I notify the downstairs tenant of any complaints and advise the upstairs tenants to call the police if they think a noise ordinance has been violated.

They have enough to lose if they don't pay or violate your lease.

You can simply alert her that her proposed guest arrangement violates building policy and that you'll be having her sign a policy agreement shortly. 

@Ernes Sahic while meeting the tenants may help you glean some information, it won’t be much, and could put you in jeopardy of fair housing violation(s), seeing their unit and the ledger for on-time payments, looking over original application and current lease, checking for police calls for service will yield much better insight, unless you see signs of an extremely bad situation, tenants are probably fine.

I've found a fourplex with $150k in violations (nothing serious, it's just been abandoned or something). 

The most motivated sellers tend to be: Absentee owners, pre-foreclosure, tax delinquent, divorce, probate, Code Violations, inherited, expired listing.Good Luck!    

I'm not ready to buy yet, but this property on 4009 Eunice St Huntsville AL 35805 (18 unit, $550k) intrigued me (also this is my first post ever so I hope I don't violate terms of use!)

You need to make sure you would not be violating any conflict of interest clause or stealing clients from your existing employer.

Tell them you are terminating because of their poor performance and/or violations of the management agreement.