The U.S. Mint manufactures coins in four locations: Philadelphia, Denver, San Francisco and West Point. Coins have borne "mint marks" since antiquity when the ancient Romans imprinted a mint's location to control the quality of coin production. Congress passed an Act of March 3, 1835, which stipulated mint marks on U.S. coins as a way to standardize production. Collectors can identify a coin's origin as either Denver or Philadelphia by looking at the year of the coin's manufacture and the mint mark.
Coins Newer than 1968
Flip your coin to the "face" side that contains an image of a person. A quarter, for instance, bears an image of George Washington.
Locate the date. If the date is 1968 to the present, keep the coin on its face and locate the single-letter mint mark, using a magnifying glass if necessary. The possible list of mint marks for coins produced after 1968 include D (Denver), P (Philadelphia), S (San Francisco) or W (West Point). Note that Congress ordered the U.S. Mint to not place any mint marks on coins from the years 1965 through 1967.
Identify the coin as minted in Denver if the coin bears a D as its mint mark. All coins minted in Denver have borne this letter except those minted during 1965, 1966 and 1967, when no coins bore mint marks.
Identify the coin as minted in Philadelphia if the coin bears a P as its mint mark and has a date of 1979 or newer. Note that Philadelphia still does not put mint marks on its Lincoln pennies.
Identify the coin as minted in Philadelphia if it bears no mint mark and has a date from 1968 through 1978. The Philadelphia mint did not use mint marks on any of its coins produced from 1968 through 1978.
Coins Older than 1965
Find the single-letter mint mark on the back of your coin. Note that the list of possible mint marks for coins manufactured before 1965 includes C (Charlotte, North Carolina), CC (Carson City, Nevada), D (Dahlonega, Georgia), O (New Orleans) or S (San Francisco).
Identify the coin as minted in Denver if the coin bears a D as its mint mark and if the coin was minted after 1863, the year the United States founded the Denver mint. The United States minted coins in Dahlonega, Georgia, from 1838 to 1861 and stamped all of those coins with a D mint mark.
Identify the coin as minted in Philadelphia if it bears no mint mark at all. The U.S. Mint in Philadelphia did not put mint marks on any of its coins because until 1838, the U.S. Treasury only minted coins at the Philadelphia location and did not need mint marks to distinguish the location of origin of these coins.
Identify the coin as minted in Philadelphia if it is a Jefferson nickel bearing any of the years 1942 to 1945 and it bears the letter "P" on the reverse side above the image of Monticello. The United States made these coins out of a silver-copper alloy instead of a nickel-copper alloy because it needed the nickel normally used for the 5-cent piece for World War II armaments and munitions, and this mint mark helped identify these coins from any containing nickel needed for the war.