The United States minted Morgan dollars from 1878 to 1904, and then again in 1921, as circulating coins. Named for the coin's designer, chief engraver of the mint George T. Morgan, the dollars were struck at five different mints--coin factories--around the country during thier existence. With one exception, each mint put its mark on the Morgans it made in the form of one or two letters on the reverse (back side) of the coin, always in the same spot.
Finding the Morgan Mint Mark
Familiarize yourself with the basic design of the Morgan dollar. The front (or obverse) of the coin depicts the head of Liberty--she's wearing a cap that says LIBERTY, besides sporting wheat, cotton and leaves in her hair. The obverse also has the date and the motto of the United States, "E Pluribus Unum," but not the mint mark. It's on the back (or reverse) of the coin, which also features the figure of an eagle, the name of the United States, the coin's denomination, and "In God We Trust."
Look more closely at the reverse side of the coin. A wreath extends most of the way around the figure of the eagle, and below the wreath are the words ONE DOLLAR, which curve their way around the bottom edge of the coin. Below the wreath and above the space between the D and O in DOLLAR is the coin's mint mark--if it has one.
Learn the different mint marks. Otherwise the letter or letters you see on the reverse will mean nothing to you. Morgans marked with S were made at the San Francisco mint; those with O at the New Orleans mint; those with CC at the Carson City, Nevada, mint; and those with D at the Denver mint, though 1921 was the only year with that mint mark. If a coin has no mint mark, it was made at the Philadelphia mint.
Also learn which of the mint marks make for rarer coins, if you have any interest in collecting Morgans. For example, 21.7 million Morgan dollars were made in Philadelphia (no mint mark) in 1889, making it a garden-variety collectible. But only 350,000 were minted at Carson City (CC) during that same year, making the 1889CC coin a much more valued one among collectors, and thus much more expensive than the Philadelphia minting.
While looking at your coin, don't remove it from its packaging, especially if it has been professionally graded. If you have a Morgan without packaging, handle it by its edges to help prevent tarnishing.