what is the best way to check the value of an old coin, when you think it might be worth a great deal of money

Answer 1

The first thing to do is to identify the coin you have. From which country is the coin? What is the denomination, the date and the mintmark(if any)? Next thing is to learn a little about coin grading. A grade summarizes the overall condition of a coin, and determines the value of a coin. You can learn about coin grading here: http://www.numismedia.com/fmv/grades.shtml
Once you've identified the coin and have some idea of its grade, a guide can be consulted for values. Some of the most commonly used coin values publications include The Standard Catalog of World Coins by Chester L. Krause and Clifford Mishler, A Guide Book of United States Coins a.k.a. The Red Book, coin magazines and monthly newsletters. You can also use online price guides, such as those provided by PCGS or NumisMedia. However, these values are retail, so expect less if you sell to a dealer.