Well, one thing that you can easily look at are the muflers or exhast pipes. I was looking into an Honda Accord, but I looked down and it had a huge whole in it. Like everyone else, I'll also say that in older cars electronic problems come into play too. Paint, was also mentioned; this 'can' determine if the car was in a fender bender or even worse, if the colors of the car don't match, (maybe the front fender is darker then rest of the body). You cannot examine the paint in the evening or when the sun is down or in a cloudy day. In a bright sunny day, move back about 30 feet or so, and look at the whole car, if the car color is dark the painted part will stick out.
You can also check if the headlights and taillights work, especially when applying the brakes. One of my friends brought a 1991 Jaguar from an auction, and he had the same problem, the taillights would turn on with the headlights but not with the brakes applied.
I see these kinds of auction as taking a little risk and the results also depend a little on Luck too. So, Good Luck.
Shah.