The very first thing you will be looking for is a smoothly-ground inner surface. If it's pebbly or rough-milled, you don't really need to get a used pan. You can buy Lodge cookware brand new and get the pebbly surface. Older pieces (before about 1960) were milled smoother.
Is it warped? Check it sitting on a flat surface by pushing down on the handle and sides to see if it wobbles. See if it spins. If it wobbles or spins, it's warped. Avoid.
Is it cracked? Inspect it visually as well as you can (sometimes it's hard to tell). Hold it by the handle and tap the bottom of the pan with a wooden spoon or other object (your middle finger will do if you flick it by bending it under your thumb and then releasing it with some force, much as if you were going to pop someone in the arm with your finger). If it rings, it's probably okay. If it just thuds, it's probably not. That said, if it's covered with as much gunk as the one I found, that probably won't work.
If it's rusty, is it pitted? Rust can be remedied, but pits can't.
You won't always find the manufacturer's name on the bottom, as some of them produced pans to be sold by stores such as Sears and Montgomery Ward. But if you do find a name, that will help identify the approximate age of the piece. The most collectable pans are Griswold and Wagner. Another good pan is Birmingham Stove and Range. There are a couple of others. But the main thing is these pans were very smooth and lighter in weight than pans made after the 60s.
What happened around that time was that other cookware was being produced that was easier to maintain, such as Corning Ware, pyrex, and Teflon. Because of that, demand for cast iron plummeted, manufacturers went out of business, and only Lodge remained. Since the smooth milling cost more in man hours to produce, Lodge stopped doing that, and that's why pans now have a rougher surface.
The little pan I bought turned out to be a Lodge, but it's smooth, and it doesn't have the Lodge logo. It was made before 1960, probably in the 40s. The only way I know it's a Lodge is the notches in the heat ring. They were the only company that notched their heat rings.
I already had several pieces of cast iron, but most of it was rough surface. The only really smooth ones I had was a set of small, medium, and large pieces made in Taiwan. Collectors don't care about those, but I can tell you they are as smooth as glass and a joy to cook in.
And by the way, if you get something that has carbon build-up, you'll need to have a self-cleaning oven to burn that off. Unless you want to soak them in a lye bath (lye is VERY dangerous, but I'm used to it, because I make soap) or set up an electrolysis tank. The pan I bought had such a layer on there, I decided to use the lye bath first instead of risking a fire in my oven. Four days of soaking and chopping at it with a screwdriver got it down to the metal, and only then did I put it in the self-clean cycle.
Just be picky about whose advice you take if you watch YouTube or read articles. Some of the information out there is downright stupid. Martha Stewart, for instance, who makes her living with all this stuff she does, should know how to season a cast iron skillet. If you do it the way she demonstrates, you will have a gummy pan, and you'll have to start over.
TMI, right? Good luck, jea!