Before the late 1980s, mint marks were hand punched into the die. So if a pre-1990 coin shows similar doubling on both the devices and the mint mark, that's a big clue to Machine Doubling.
Also, I believe that proofs were struck twice in the 1960s. Don't quote me on that, though.
Minor Machine Doubling. A true Doubled Die would enlarge the devices, not reduce them in size like Machine Doubling would. Proof coins are struck more than once, so the chances of Machine Doubling happening is more likely.
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