How do you tell if estate sale pliers are worn out or just dirty?

Sounds like you did fine. Stiff joints and a little surface rust at the pivot are just what happens when tools sit in a drawer for years, and it wipes off with oil the way yours did, that's honest wear, not junk. I've bought plenty of used pliers in worse shape and most of them cleaned up and cut fine for years after. For the cutters, yeah, just test them on scrap. Grab some copper wire, romex scrap from a cutoff works great, and try a clean snip. A good cutter shears it with a crisp pop and leaves a flat, bright face on the wire. Worn or nicked cutters will roll the wire, leave a pinch mark, or mush the end over. Do that a few times at different spots along the jaw since wear isn't always even. Also close them fully with nothing in the jaws and hold them up to the light. A thin line of daylight down the middle means the jaws are misaligned or worn, which matters more for crimping and gripping than cutting, but it tells you how much life is left. On the vinegar, it works but go easy. Dilute it half with water, soak for maybe 30 minutes to an hour, then scrub with a Scotch Brite pad or old toothbrush. Left too long it'll darken the steel and eat into any plating, so check on it. The important part is what comes after. Rinse well, dry it completely, hit everything with oil right away or you'll get flash rust within the hour. Honestly though, if the rust wipes off with a rag and oil like you said, skip the soak. A rag with some 3 in 1 oil or even WD-40 and a nylon abrasive pad will handle light surface rust fine. For next time, work the pivot on every pair before you buy. Check for side to side play in the joint, look at the teeth on slip joint and water pump pliers for rounding, and peek at the brand stamping. Vintage Channellocks and Knipex and old Craftsman hold up way better than the no-name stuff, though at your price point it's hard to go wrong either way.

This thread is helpful, I found a similar pair of cutters cleaning out my dad's garage last month and had the same worry about them being junk. The scrap wire test is a good idea and I'm going to try it this weekend. One thing that worked for me on the stiff joints was putting a drop of oil right in the pivot crack and working the handles open and shut a bunch of times, then wiping the excess off. Took a couple rounds but they loosen up quick. Do you all have any trick for getting oil down into the joint without making a mess everywhere? I ended up with drips on the workbench and had to degrease the handles after.

Gotta push back on the light test though. A gap down the middle isn't automatically a red flag, diagonal cutters pass each other when they close and are never supposed to touch, so you'll see daylight on a perfectly good pair. That check only really tells you something on pliers where the jaws are meant to meet flat, like water pumps or slip joints. I had a pair of old diagonals I almost gave away becuase of that test, then snipped some scrap wire and they cut cleaner than anything in my box. If the wire test looks right, don't overthink the gap.