Test drive. If something seems off, check.
How to check.
1. Check compesson (should all be close numbers aross all 4)
2. Check Oil (see if anything bad is in the oil, check if low)
3. Ask questions. He might be a shade tree type.
4. Start car, with raditor cap off, let warm up, and see if there is bubbles coming out of the raditor. Also, make sure there is no oil in the raditor fluid.
What I have learned over the years, the body poorly maintenanced, the engine poor maintenance.
Problems.
1. Would be Rings
2. Could be alot of things. (if anything shows)
3. See if he is a shady guy, if so, dont buy it.
4. Head gasket