CarFax helps a lot but is not foolproof. Check for mismatched or absent pinstripes, brand-new tires or wheels on one side of the car but not the other, texture and paint under the hood, differences in paint texture or color on adjacent panels, paint run-over marks along masking-tape edges, uneven panel gaps, pulling to one side or another on a level road, new grilles and radiators, etc.....
As a part-time auto consultant, I have seen a lot of junk on dealer lots masquerated as good used cars....though the problem is much better now than it used to be.