Old coins can be extraordinarily valuable. Oftentimes older coins are made of precious metals like gold and silver, making them extremely valuable, even if you disregard their historical context. And often times their historical context serves to make them even more valuable. The worth of older coins depends on a lot things, from the physical quality of the coin to how old the coin actually is. A good way to find out the value of coins is to locate a coin price guide.
Figure out what type of coin you have. Is it an American half dollar? Is it a European ducat? Usually, the country where the coin was minted is written somewhere on the coin itself, as is the denomination. If you cannot figure out what type of coin you have, a dealer will likely be able to help you. Once you know what you are looking at you will be able to determine the value of your coin.
Find the date the coin was minted. This almost always appears somewhere on a coin (usually the front). Older coins are generally worth more than newer coins.
Check to see if the coin has a mint mark. A mint mark is small symbol--usually a letter--that tells you where the coin was minted. Not all coins have mint marks, but if they do, they oftentimes affect the value of the coin.
Ascertain the physical quality of the coin. Coins are graded on a system that runs from fair (the lowest grade, sometimes known as poor) to uncirculated (the highest grade). An fair coin will be well worn, and the lettering, date and pictures may be difficult or impossible to read. An uncirculated coin will look flawless. Special proof sets exist, but these are specially minted coins that never reach circulation.
Look up the value of a coin in a price guide. Price guides are the easiest way to find out the value of a coin, and they tend to cost less than ten dollars. There are multiple price guides on the market, many of which focus on specific kinds of coins.