This may vary from state to state but here in CA you can call the Assessor's Office and ask them. You just have to give them the address and they give out the registered owners first and last name and mailing address. They don't give the phone number but if you Google the person and hunt around you can usually come up with a number.
I see you're in Philly so here you go... good luck.
Outside of the tax assessors website the easiest way is to just ask the owner. “What is your email address and phone number?” Seems to work for me