Because many American collectors collect coins by date and mint mark, the presence of a mint mark can (and often does) change the value of a coin considerably. This is because it can affect the rarity of the coin. For example, in the case of 1894 dimes, having the 'S' mint mark adds tens of thousands of dollars to the value of the coin as only 24 were minted.
Many American coins lack a mint mark. That is because in the beginning, there was only one mint, Philadelphia. So any coin lacking a mint mark is from the Philadelphia mint (or extremely recently, it might also be from West Point, but there is no way to tell.)
The following mint marks exist:
The following list provides a hint as to where to look for the mint mark. Any coin not listed was probably only made in Philadelphia and thus lacks a mint mark.
Indian Head Cents - 1908-09 only (S) on the reverse under the wreath.
Lincoln Cents - On the obverse under the date. (D,S)
Three cent silver - 1851 only (O) to the right of the C
V Nickel (1912 only (D,S)) - Under the dot to the left of the date.
Buffalo Nickel - (D,S) - Under the "Five Cents" on the reverse.
Jefferson Nickel - (D,S) -
Mercury Dime - (D,S) Reverse to the left of the bottom of the faces.
Roosevelt Dime - (D,S) Reverse to the left of the bottom of the torch.
Standing Liberty Quarter - (D,S) To the left of Liberty's feet on the
Washington Quarter - (D,S)
Walking Liberty Half - (D,S) 1916-1917 Obverse under "In God We Trust"
Gold Coins - In general, look under the tail of the eagles or wreaths
on the reverse.
Incuse Eagle - To the left of the arrows under the eagle on the reverse.
St Gauden's - Above date (The A/G under the date is the designer's initials)
Commemoratives - Just look around the coin carefully. Remember the mint mark is generally in the field, not part of the device. Possible mint marks are (D, S, W).