Where is the D on the 1916 dime?

If you have a so-called "Barber" dime (named for its designer, not a hair cutter) there won't be a D mint mark because that design was struck only at Philadelphia and San Francisco.

If you have a Mercury head dime the mint mark position will be to the right of the E in ONE on the back. It may be blank (Phila.), or there may be an S or D.