Instructions
Conduct a visual inspection of the coin first. Oftentimes, you will not have an opportunity to handle the coin before the purchase, as is generally the case when you're buying from an online vendor. Look at the date; if it was minted after 1964 then it is likely copper-nickel clad.
Do some research before you buy any coin. Look to the "The Official Red Book," which has information on every coin ever minted in the U.S. Additionally, the "Red Book" reveals when a coin was produced, how rare it is and its current market value. The book (a new edition is released annually, so ask for the current edition) also features photographs of many of the coins.
Browse for more information online by searching various Internet coin databases, like World Coin Gallery. Also, visit online coin message boards and forums and ask questions about the coin you are trying to identify.
Bring your coin to a professional coin dealer to help you identify the coin. Furthermore, the dealer may be able to share some insight on how to identify other rare coins.