First thing I look for is a BBB report. (Better Business Bureau)
Secondly I talk to people for "word of mouth" reputation.
Thirdly, low miles on a vehicle, cleanliness under the hood as well as the underframe area.
Fourth, pay attention to the salesman. Why is he pushing this car on you? Why does he evade certain question on this car? Is he trying to quick sell the car or volunteer you the time to think it over?