coin knowledge dime

I know this is probably a stupid question, but here goes; How do you determine a 1968 S mintmark dime is a 1968 S mintmark
which is missing the mintmark. In other words, if you have in hand a 1968 dime with no mintmark which is supposed to have the S, what determines that it is missing the S?
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A 1968 dime minted in San Francisco would be a proof coin.
A 1968 dime minted in Philadelphia would be a business strike.
If a San Francisco dime was missing the S it would still be a proof coin.
The proof coin should have surfaces which are polished and very shiny like a mirror.
The business strike coin surfaces are less shiny - not polished.
Above photos are from PCGS CoinFacts.
So any 1968 proof dime I find will always have an S, unless it is missing, in which case it will be the missing S dime!
By Jove, I think I've gawt it!
A proof coin, you'll know it when ya see it 😉