PA online casinos are required to keep and maintain geolocation verification software as part of their online platforms. The software allows them to pinpoint a user's location at any point during play.
This software mostly uses the inboard GPS found on mobile devices in order to work. However, should you use one of the few sites that will allow play through browsers, you will need to verify your location through a separately-downloaded geolocation app called GeoGuard.
Geolocation software is designed to err on the side of caution when it comes to verifying a person's location. You are more likely to be falsely denied service than you are to circumvent the system from out of state.