Pretty much the best you can do is to test ride it up and down the street.

There are quite a few members here, always look out for the good findings and posted their treasures in this forum. You kind have to take time to look around and build up your instinct. For example, if you see dust on the bike, not kind that builds up when you ride it but kind of dust that builds up on everything in the garage. That usually means owner just lost the interest in cycling and the bike had stayed in the garage for a long time. And it's always good to ask the seller if the bike has any damage. Most owners are honest about it. Even if they are not honest, most damages are visible. It's not like a car, you can't hide the damage so easily. If you feel that the seller is not honest, leave it no matter how good the deal seems to be. You'll never know what else the seller is hiding behind.

And it never hurts to have a good knowledge about the makes, models and the components. You can find a Mart bike for $50.- at one garage sale, and you'd find another bike at the next garage sale originally priced at $600.- as a new bike, sold at also $50.-.