Where do you find the S mint mark on a Kennedy half dollar?

Except on the 1964 issues where the mintmark was located on the reverse of the coin under the olive branch that the eagle is clutching. The mintmark is located on the front of the coin, below Kennedy's face and above the the space after the first 2 numbers of the year. Keep in mind that your coin may have no mintmark, in which case it was struck at the Philadelphia Mint.