Like so many of the nearby states in the eastern part of the country, Pennsylvania received placer gold from ice age glacial drifts. You will probably not find sizable quantities of gold in the state, but a hobbyist may find enough to make it worth his time. This sort of gold is frequently widespread and difficult to locate. There don't seem to be rules to follow – just keep looking at different areas until you see some color.
York and Lancaster Counties are the most well-known counties for placer locations, but very small amounts could probably be found throughout the state. The areas near Grantham, Wellsville, Dillsburg and Rosville, in York County, have gold in most streams if you can find the right spot. To the south Winterstown and Shrewsbury are worth a look. Just remember that this gold is going to extremely fine in texture.
In Lancaster County to the east, gold has been found as well as platinum nuggets in the Susquehanna River and Peter's Creek near Quarryville. Also worth a look is Wyoming County in the northeast, which has several creeks with placers.
While there is a sizable mining industry in the state that has produced some gold as a byproduct, iron and coal are the primary minerals being mined. So take a gander around the mines near Morgantown and Cornwell, because any time there is lode gold production at all there will generally be placer gold to be found in nearby waters, even if in small amounts.