How do you tell the difference between a denver mint peace dollar and a philly one?

On the back of the coin, under the ONE in "ONE DOLLAR", there will be a "D" if the coin was minted in Denver and if there is no letter under there it is a Philadelphia minted coin. Your coin may have an "S" as a mintmark rather than a "D" or no mintmark, in that case your coin was minted in San Fransisco.