How do I look up a contractor's license status in Pennsylvania?
It turns out that licensing for plumbers (and other contractors) in the city of Philadelphia is separate from whatever licensing concerns the rest of the state of Pennsylvania. I ended up calling the city's 311 information line and they were able to search their database of licenses for me (it turns out this guy is not licensed).

The city does maintain an online database of licensed professionals of several types (including plumbers) here: http://www.phila.gov/li/Pages/FindLicensedProfessional.aspx. But from what I was told by the city, that list is not comprehensive, only contractors who agree to appear there show up. So if someone is listed there, they are definitely licensed, but if they aren't listed, that doesn't necessarily mean they are not licensed. For a definitive answer, you need to call the city's 311 line.