Make an appointment on the day you will look at the car. Advise the owner you will have the car inspected. Decide on the exact make and model before you buy. Then research the car in magazines and consumer reports. Learn the history of your car.
Test drive many cars, especially the beat up ones.
Test drive as many used cars that are the same make and model as the one you plan to buy as possible, especially cars that are so used and worn you would never consider buying them. By test driving many cars, you are very likely to detect any recurrent mechanical problems, then see if you notice similar symptoms in that gem you are looking at. Talk to people with the same car. If you see someone with the same car you want to buy, ask them how they like it. Most people will be happy to tell you.