The value of old coins is determined by a number of factors, not the least of which is the rarity and condition of the coin. Once you have an idea of the condition of your coin, you can look up its value in a coin price guide. If you are having trouble with any aspect of this process, it is a good idea to take your coins to a coin store and speak with a professional dealer.
Classify your coins according to country of origin and denomination. This information is usually found somewhere on the coin itself. If it is not, you may need to do some research. The best place to start would be your local coin shop.
Find the year the coin was minted. This is usually on the front of the coin, but sometimes it appears on the back. Roman numerals are occasionally used to mark the date on coins.
Look for mint marks. Some coins have marks that designate the specific location at which they were minted. This mark can greatly affect the value of a coin. Different types of coins have mint marks at different places, so make sure to thoroughly check both sides of your coins.
Grade your coin. Every coin has different criteria upon which it is graded, so you will have to read up on the specifics of your particular coin. This info can be found in most price guides. The grading for circulated coins runs from "about good" to "uncirculated."
Buy a price guide. There are many quality coin price guides on the market, like the "Blackbook Price Guides." These guides can be found in almost any coin or book shop.
Look up your coins in the price guide. Make sure you are taking all of the coin's factors into account when doing this.
Take any especially valuable coins to a dealer to be professionally graded and appraised. By doing this, you will find out the exact value of your rarest coins.
Grading coins accurately takes many years of practice. When pricing your own coins, it is acceptable for you to use approximate grades, but be aware that your grades may not be accurate. A good way to learn grading is to spend time in coin stores examining professionally graded coins. Remember that the value in the price guide is not necessarily what you will get for your coins should you choose to sell them. The price guide acts as a reliable guide as to the value of your coins, but real world trends can affect your coins' value. Online coin price guides are a convenient place to start determining the value of your coins, but they are not as well respected as print price guides.
Cleaning coins is generally frowned upon. Dealers prefer to buy coins in their original state. If you do clean your coins, you will have to inform any potential buyers of that fact.