vin number

Usually there is a vin on the dashboard that you can see through the windshield on the driver side. You can also check any old registrations you have for the car, or insurance cards. In CA at least, that number appears on both currently. You might be able to call a company that has insured the car in the past and see if they have it. And the DMV might be able to look it up from a license plate number (in some states you can get that online from the DMV website looking up the plate). Good luck.

If you don't find it there here's a few more places to look: stamped in metal somewhere on the frame, riveted to the dash.