How do you know if a Roosevelt dime is missing a mint mark how do you know where it was made???

[BronerskyFamilyTrust]

I have been reading up on Roosevelt dimes. An article about 1968 San Francisco mint mark error, missing on the 1968 dime.  My question is, if you have a Roosevelt 1968 dime and it has no mint mark, does this mean its Philadelphia or the rare error? In other words, if a coin is missing  a mint mark, how do you know? how do you know its a San Francisco mint error?   So confusing.

[Conder101]

The rare 1968 "no mintmark" dime is supposed to have an S mintmark. All 1968 S dimes are proofs. Proof dimes are made by a different process than business strike dimes. A proof dime will have a much sharper strike, sharp square edges that meet the rims in roughly a 90° angle, and mirror finish fields. They may or may not have cameo contrast between the devices in the fields. Business strikes to not have that mirror finish field. Even with moderate circulation the sharp square edges and mirror fields of a proof will still be evident. Best thing I can suggest, is go on eBay and by yourself a 1968 S proof dime. Even with shipping you should be able to pick up one for a couple dollars. Once you see it in hand you will understand the difference, and you'll have a comparison piece to compared to other 1968 dimes that don't have mintmarks. We can describe the differences all we want, and even show pictures, but it's just not the same thing is actually holding a proof coin in your hands. Once you see them you will understand.