Man, it is really impossible to tell what to look for. There are many things and some of them are really difficult to know for sure. Even a through inspection may not guarantee things. You may drive the car for thousands of miles w/o a single issue or your dash board lights start coming on one by one within a few weeks.
Buying a used car is always a risk in my opinion. But most of the time it is a risk worth taking. Otherwise nobody would buy a used car.
I am driving my 4th car. None of them were new...