The 1896-S Quarter Dollar is a well-recognized rarity in all grades.
All 1896-S Quarter Dollars show the mintmark positioned above the space between the R of QUARTER and the D of DOLLAR. However, there are two different dies with this feature: one with the S low and close to the R; another with the S higher and farther away from the R. To see illustrations of both, click on the "View All Images" link. If the mintmark is not in either of these positions, or does not have the same shape and appearance as the ones illustrated here, then the coin is most likely a counterfeit or the mintmark has been added.
Sources and/or recommended reading:
"Those Shifty Mintmarks" by David Lange, The Numismatist, September 2010, pp. 21-22