Year, Model and Specifications
The most obvious one is the Year and Spec of the car itself. Once you've found yourself a car, it wouldn't hurt to do a little history look-up on the dealer themselves. A lot of dealers these days claim their cars are of later models and of higher spec. I've seen a lot of GTR's who claim to be of 2002 but are not. Claimed to be of V-Spec or V-Spec II or even V-Spec II Nur but are most definitely not. Everyone wans to get what they pay for so be careful and be on the look out.
Usually, you can more or less tell what year the car was manufactured by looking at the sashes of the seat belts as there would be an indication there. But unfortunately, shoddy car dealers now cut those sashes off as consumers are becoming more aware.