A professional appraiser is one of the most reliable resources to determine the value of gold coins. The American Society of Appraisers and the International Society of Appraisers both have directories of professional appraisers, according to prattlibrary.org.
There are a variety of resources available to learn about gold coins and currency. The "Red Book" formally known as the "Guide Book of United States Coins," is a solid source of information regarding U.S. coin values. A collector can also join the American Numismatic Association and have the advantage to learn about U.S. currency and stay up-to-date with new discoveries in the industry.