There's no guarantees with used cars, but you're mostly looking for:
Appropriate miles for the years. Too few miles will likely mean the car wasn't used often enough (some parts do not like sitting unused). Lots of miles and there will be suspension wear that's hard to see (although, it can be a good sign for the engine as long trips are MUCH easier on an engine than city driving). 10,000-15,000 miles a year is considered normal for cars in the USA.
Rust. Look carefully under the wheel arches and on the bottom of the doors. Check the bottom of the car as well. Some surface rust is expected under a car of age, but look for weak spots/holes.
Is the car straight or is it crooked? Should drive straight and body should look straight.
Receipts. It's not necessarily bad if someone doesn't have them (I don't keep mine, and I'm obsessive about maintenance!), but if someone DOES have all the maintenance receipts you can check and see how good they were about keeping up on routine maintenance, as well as any problems that might have been happening.