No general accept method. IMHO in most peoples mind once restored the original mileage (or even after a repaint for example) doesn't carry allot of value or importance
I've seen people use insurance paperwork over the years if the company collected that info every year. Or if your state that collected printed it on the registration or when it changed hands. The problem is that if you have a break between maybe 4-5 years or more these records can mean little as an owner can use the car and put on an extra 100K miles between the recorded records.
I've had a Calif Special that had about 50-70% original paint and looked like your normal Calif car. Often you can't tell a car with 40K or 120K miles so in inspecting the car I figured it was mid 100k - Can't recall the exact mileage. After I purchased the car and went through the whole big file the owner gave me (never bought a car where the owner kept records out here - just another old car) sales and service receipts suggested over 300K on the car.
Just an example - your normal may vary
