Test drive

Try to test drive a car for as long as you can. Try to accelerate, decelerate, take it on the highway if it's possible. The more you drive, the more chances you discover possible problems. If it's your first car, try to test drive few different cars of the same model to have better idea. If you feel anything that may indicate possible engine problem (e.g vibrations, stumbling, misfiring, delay during acceleration, rattling noise, smoke, rough idling, etc.) avoid buying such a car. If you have any hesitation about the way the car drives, shop around, there are plenty of used cars available. Sometimes a sales person may try to push you to buy a car today and now because "tomorrow I won't give you this price" or "I have the customer who will buy this car tomorrow" - Don't rush, take your time to think, the more you shop around, the more chances to find the right car for the right price.