If the dime is not a Proof coin, then it is likely that it was minted at the Philadelphia Mint and would not have a Mint mark. There were several occasions when coins Minted at San Francisco did not bear Mint marks to insure maximum circulation (i.e. that the coins would not be taken out of circulation by collectors) but these would prove almost impossible to identify and they likely have no more value than a Philadelphia Minted coins.
Dimes minted at Philadelphia didn't carry a mint mark until 1981. There were something like 424 million dimes minted at Philadelphia in 1968, so it is just a normal dime.