Before you even start the car, look under the hood and see if the engine feels warm. People, who have cars that are hard to start while cold, will often times take the car out and drive it to operating temperature. The car will then start up easily for the prospective buyer, when in actuality the car doesn't start right up for anyone and requires a lot of time and trouble to get the car running. A car that is hard to start on a warm morning will be next to impossible to start on a cold one. Another trick that some will try, is if the transmission is missing the reverse gear, the seller will roll the car into a driveway backward so the test driver never has to use it.
Most people forget to test all the gears in a prospective car because they simply overlook it.
Check the fluids on the car. Make sure there are no foreign objects in the oil or the transmission fluid. The oil should be clean and free from any water or gasoline. Any of this found in the oil can indicate an internal motor problem. The transmission should appear to be reddish in color and should have no water or metal fragments in it.