What I do is look inside the radiator and see if its pretty clean. If it is, then I check to see if it's original by carbon dating the bugs in the fins. This is not "fool" proof.
Seriously, if you had a murder case and wanted to prove you had 69,000 original miles, you may be able to tell, under extreme microscope, if the 7 or 8 or 9 had ever been exposed to sunlight or their gear tooth had ever been engaged.
