Appearance, test drive, seller, etc.
I've always gone with the dirty car outside, dirty car internals. I also use the windshield washer fluid test...if the seller can't be bothered to fill up the windshield washer fluid, I am on red alert for the rest of my inspection/test drive.
Unfortunately the $2000 car is the $500 car of a few years ago, at least around me. So you have to wade through a lot of junk to find the gems. Best thing to do is bring along a knowledgeable friend that has the same model car you're looking at to help point out problem areas. But again, you're not going to have a lot of negotiating room, if it runs and drives, someone is going to buy it, and probably pretty quickly.