It is easy to get confused about mint marks. Many coins have a mint mark on the reverse or obverse or have no mint mark at all. Not to mention, each denomination may have different mint mark placements.
First, we must cover what the difference is between the obverse (Front) of a coin and the reverse (Back) of coin. On most U.S. coins the obverse (Front) will have the bust of a famous person i.e. a president, or famous person, and earlier coinage it's usually a woman's bust with exceptions like the flying eagle that have an eagle on the obverse.
Now, the reverse (Back) can have an Eagle in some form or a memorial as in the Lincoln Cents from 1959 to the present. Another aspect of the reverse is the denomination. The denomination of the coin is how much the bank or government says the coin is worth i.e. one dollar, quarter dollar, one dime, etc. The reverse will have this information near the bottom of the rim.
That should be enough information to identify the obverse(front) and reverse(back) of a coin. With this information fresh in you’re mind, I can now tell you a little about mint marks and their placement on coins.
Many of the earlier U.S. Coinage stamped the mint mark on the reverse up until the Lincoln Wheat Cents. It is these cents that began the stamping of mint marks on the obverse and under the date. In the present all mint marks are on the obverse with only a few exceptions like the American Silver Eagle.
If a coin has no mint mark then it was struck at the Philadelphia Mint and some coin enthusiast designate this with a (–P) i.e. 1950-P. Some coins do have a “P” stamped mint mark, but most don’t, the Jefferson Nickel is but one example of a denomination having a P mint mark.
It is important to know this about the P mint mark, because many early coins don't have a mint mark, and this means they where struck at the Philadelphia Mint and can be designated as “Plain”or with a P as stated above.
There's several U.S. Mints that struck coins over the years and some no longer exist, below is the list:
Philadelphia, Pennsylvania Mint- coins from this mint usually don’t have a mint mark if they do it will be a P.
San Francisco, California Mint- coins with a S stamped on the reverse or obverse.
Denver, Colorado Mint- coins with a D mint mark on the obverse or reverse.
Dahlonega, Georgia Mint- this mint also used a D and is stamped on gold coins from 1838-1861 only.
West Point Academy Mint- Coins with W mint mark. Only the American Silver Eagle (beginning in 1996 to present), Modern gold coins, and the 1996-W Roosevelt Dime.
Carson City, Nevada Mint- coins will have a CC mint mark on the reverse.
New Orleans, Louisiana Mint- coins will have an O mint mark on the reverse.
Charlotte, North Carolina Mint- is a C mint mark and is only on certain gold coins.