How to find the owner of a property you are interested in...

[James Brand]

While looking into one particular property I was interested in, I noticed another with seemingly identical bones. This other property was a more run-down version definitely requiring more rehab and likely vacant. The property is not listed for sale but given its state I would imagine the owner would entertain an offer. Can anyone guide me in how I could go about ascertaining who owns the property? I am in Philadelphia, PA, and this is a Duplex...