Get the vehicle identification number (VIN) as well as the make and model of the vehicle that you're trying to find. This is the basic info that you're going to need for your license plate search.
Find a reverse look-up program that will give you the license plate number for details about the vehicle. Abika has such a program for those who want to find a specific vehicle anywhere in the country, including Pennsylvania. (See Reference 1)
Type in the vehicle identification number. There may be requirements, depending on the program you're using, for make and model of the vehicle as well, but the VIN is really what you need. Once you have all the information that you know hit the "Search" function. The program should then look up all the vehicles that meet your search parameters, and give you their license plate numbers.