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.
Take your coin to an official dealer. They are professionals, and they can usually tell you a great deal about the type of coin, its condition and its worth.
Coin grades are as follows: fair (also known as poor), about good (also known as almost good), good, very good, fine, very fine, extremely fine, almost uncirculated, uncirculated (also known as mint state). Grading is very complicated, and each coin has different details you must look to determine the grade. Details on how these grades are determined appear in any coin guide. There are also many web pages that can help with grading.
Grading coins takes a lot of knowledge and practice. It is always a good idea to take an especially valuable coin to a dealer for an official grade.
After finding the value of your coin, it is a good idea to see if you can find what that type of coin is selling for at auctions and at coin shops. Coins don't always sell for their book prices.
There are several quality price guides on the market, including the "Blackbook Price Guides," and the "Red Book Guide to United States Coins." There are a few online price guides, but they are not as well respected as their print counterparts.