Silver coins, especially those produced by the United States Mint, are currently only offered as Bullion or Proof coins. This is because precious metals such as silver are no longer used in the minting of currency that is put into regular circulation. As a result coins that were once made from silver can sometimes be hard to identify from coins that do not contain silver. Coins that are produced today look very similar to a silver coin but the composition is different.
Know when the United States Mint stopped using silver in coins that were minted for regular circulation. The Kennedy Half Dollar is an good example--silver was used in the composition of the coin until 1964.
Look online at sites such as the Professional Coin Grading Service to information about various types of coins. This site has information about various types of coins, including older coins that were made from silver.
Check the edge. If a coin that has any traces of a dark copper color along the edge, it is not a silver coin.
Find out what metals are used in the composition of non-silver coins. The composition of coins today are made from a variety of metals such as zinc, bronze and nickel.
Avoid collectors mints that produce coins that claim to contain silver. Many collectors coins from places such as the Franklin Mint and the National Collectors Mint are reproductions and are not even United States currency.
Buy a coin guide or book for easy reference. A good recommendation would be the Red Book of United States Coins.