1. The car's repair and maintenance reports. Especially these are important to know if the spent km is real or it has been played "tuned".
2. To check the paint with special device to understand if the car has an accident. If possible get information from police or insurance companies.
3. To check mechanical status of motor and gear box. A drive test, acceleration and gear shifts can give you an idea.
4. Check if the car makes too much not expected noise in the notched roads.
5. Ask as many questions as you can to the seller and try to understand the cars history.
6. It would be good to go to a registered car service to get complete inspection report.