Things that I look for...
1. Previous wreck history or paint/body work. (rough edges around the inside of the door jams, different shades of paint, excess or not enough of "orange peel" in the paint, panel gaps, dents..ect. I've even brought a paint thickness meter to look at cars.
Amazon.com: Automotive Paint Thickness Meter Gauge Gage for Body Shops and used Car Dealers: Automotive)
2. Mods past or present (missing bolts,broken clips or loose parts, ect)
3. Unusual mileage or wear... (low mileage car with worn or wrong replacement rear tires, excess driver seat wear, 2010 MY car but with 45k miles)
4. Documentation. (receipts for past work or service, warranty paper work or dealer ship invoices ect)
Ask questions... try to feel out the owner, ask why they are selling, ect ect.
Do your homework, find out if there is anything wrong with the previous year or model as far as recalls or TSBs.
I did my research on the 370Z about 6-8 months before I bought mine. I got a killer deal and a great car because of it.