Shape. Color. Flaking. Knowing the most common type of lithics used in your area of Pa. Visit area museums if possible, and collections, online, or in person, to understand better all those things. Especially, I believe, getting to the point that when you spot even a little piece of the lithic material common to your area, it will stop you on a dime. That takes getting out there, and practice, practice, practice.
Obviously, look in the right places. Cultivated fields, with permission, that are near fresh water sources, and after heavy washing rains.
Here's what will happen eventually. You will get to the point that recognizing a possible artifact using material recognition, flaking, etc will become second nature. When that happens, recognition of a potential point, or whatever, will become near instantaneous. You won't be saying "that's the right color, I see flaking, I like that shape". Yes, on an unconscious level those thoughts will be there, but once you have enough experience, it will all be at a second nature level. You'll still be fooled by the shape of a leaf, a sherd of glass, whatever. Happens to all of us, but the recognition of what you need to recognize will actually happen very quickly, at an unconscious level, and you will jump on stuff even when only a tiny portion is showing. Like some of the in-situ photos above demonstrate must have been the case in order to find those hard to spot points. That guy had experience under his belt to have spotted those. It's probably second nature by now for him to do that. Walk and practice. For hours on end. It's a learning curve, but it's not brain surgery, lol, you can get to that point just like we all have.....
Good luck!