@Jon Ostojic  first I suggest you make sure the location is good and that the actual rents and occupancy are correct and verified. 

Will you have the opportunity to look at the owners' books to verify rents (are tenants actually paying?)

The property is technically two identical duplexes right next to each other on one tax parcel of land.

My question is, how do I verify that they are cash-flowing?

I always like to do this just to verify that the pro forma and projected rents are realistic.  

Nobody can guarantee you will win in court but it's normally pretty easy as long as you can verify your story with documentation.If you take the tenant to court and evict, you will also be awarded a judgement for whatever the tenant owes.

The only thing that gives me pause that I'd want your father to verify is whether the property is on the Schedule E or A and what kind of heartburn that causes the accountant.

Prescreen before you show the property and once they apply do a comprehensive screening (background, credit, eviction report) Make sure to take the time to call landlord references and verify employment. 

In addition, take the time to call landlord references and verify their employment. 

Zillow will call your phone number the next 3 days to verify the ownership and speak with you over the phone before they present an offer to you.