Forgot to add, on other vintage of seated liberty coins it can be inside the wreath instead of below.
Also, as a general rule of thumb, most silver coins (and most non-silver older coins too) have the mint mark somewhere on the reverse (tails) side, near the bottom.
If in doubt, look at the bottom center or bottom left.
The mint marks on most larger denominations went to the heads side below or to the right of the head in 1968 after they switched over to clad. (1965, 66, 67 coins didn't have mintmarks)
For example:
All seated half dimes, all seated and barber dimes: usually below wreath, sometimes inside wreath.
All seated 20cent pieces, seated and barber quarters and half dollars: below eagle
Mercury dimes: bottom left of fasces (bundle of sticks)
Silver rosies: bottom left of torch
Washington quarters: below eagle
All walking liberty halves besides 1916 and some 1917: left of eagle
Franklin halves: above bell
1964 kennedy halves: left of eagle
Morgan dollars: below wreath
Peace dollars: left of eagle, below "ONE"
1851-O 3-cent piece: to right of the big C with the "III" inside it. (not easy to tell which is the tails side, I'm going to call this the tails side since the date is on the other side.)
exceptions
Standing liberty quarters: on heads side at left of date
1916 and some 1917 walking liberty halves: below "in god we trust"
40% silver Kennnedy halves 1968-1970: below kennedy's head (this is in keeping with the general change to putting them on the heads side after 1968, but the kennedy was the only circulation coin made with silver content after 1964, and only for these 3 years, circulation coins after 1970 don't have silver content) (silver proofs and collector coins of various types don't count, since they weren't made for circulation)