Where is the S on a 1908 Indian head penny?

IF the coin has a mint mark, it would be on the back under the bow of the wreath.

But remember, only about 1.1 million 1908 cents were minted at San Francisco compared to over 32 million at Philadelphia (no mint mark) so it's not likely that the coin will have a mint mark.

Interesting trivia: 1908 and 1909 Indian head pennies were the only US cents to ever have a mint mark on the reverse side.