How's the body? Hows the paint? How's the interior? How are the tires? How well does it run? Which trim line? Which optional equipment? You could use the book definitions of outstanding, clean, clean, average, rough, or damaged as a rough estimate. The best way in my opinion is to start with the value of a car in outstanding condition and begin deducting the cost of everything needed to bring the car of interest up to excellent.
Outstanding - Exceptional mechanical, exterior and interior condition; requires no reconditioning.
Clean - Some normal wear but no major mechanical or cosmetic problems; may require limited reconditioning.
Average - May have a few mechanical and/or cosmetic problems and may require a considerable amount of reconditioning
Rough - Several mechanical and/or cosmetic problems requiring significant repairs.
Damaged - Major mechanical and/or body damage that may render it in non-safe running condition.
