How to confirm who property owner is?

I believe I previously asked this question but was not satisfied with the answers as they seemed to miss my question and situation. I will attempt to be more clear this time.

Advice is given here to confirm who the owner is of the property your applicant is (was) living at so he can’t give you the phone number of a buddy who will stand in for the actual landlord. I have a situation with one of my properties that may make that sort of verification difficult for the other landlord. So I was thinking, if I came upon a similar situation how might I confirm who the property owner was.

Situation: a single parcel of land with a duplex on it. The city has assigned separate house numbers to each unit. One of those numbers is the same as the property address but the other number does not tie to the property. That is, the property is 617 Main Street and the units are 617 Main St. and 619 Main St. If I go to the County Assessor’s website and look up 617 Main Street I get information on that property. If I go to the County Assessor’s website and look up 619 Main Street I get nothing. It is not a valid property address. I even telephoned the Assessor’s office and they had no info on 619.

If the property the applicant is coming from is in my city then I could drive by and can see that it is a duplex and what the other address is. But this duplex gets a number of college students who are from outside the area; sometimes another state so this would preclude me driving past.