If purchasing a used car from a dealer, you'd want to see if there is a warranty or limited guarantee if something mechanical goes wrong. If purchasing from a private party its always a good idea to have a mechanic look it over prior to purchase. Otherwise look for check engine light on dash, abnormal wear on tires can mean there are issues with suspension components. Take vehicle for a test drive to see if brakes and drivetrain all seem normal, operate all interior switches for windows, heaters, etc..., check underneath vehicle for any major leaks, check fluid levels, check for any type of body repairs which means vehicle was in an accident. A Carfax report is always a good idea.