Instructions
Do your own research first. Many price guides are available to help you get a ballpark figure for your coin's worth. Without expertise, a rough estimate may be the best you can do. Different grades (conditions) make for different values; as a coin's grade gets better, its value increases, especially at the higher grades where the tiniest imperfection can change the value dramatically.
Take a look at coin magazines to find out what your piece and similar coins are selling for. Official price listings give you a figure to work with, but as with anything valuable, a coin's worth is only as much as people are willing to pay for it.
Network with collectors and collecting groups. Both online and local groups can provide valuable insight and advice regarding appraisal methods, though locals have the advantage of being able to point you toward a reputable dealer who can honestly and efficiently help you find your coin's worth.
Find out if your coin dealer belongs to a collectors' organization. Groups such as the Professional Numismatists Guild (PNG) hold their members to strict standards and a code of ethics which, if violated, puts the dealer at risk of being suspended or expelled from the group.
Get in touch with multiple dealers. Even in cases where you have honest sources, opinions and knowledge can vary from one person to the next. Should your coin be valuable, you may get a better offer from someone looking to complete a collection or someone who has a fondness for that type of piece.