Just go to your local Gun Shop and ask them what they will give you for it. Whatever they say you can be sure it is worth double that /ubbthreads/images/graemlins/yellowlaugh.gif.
Or you can look it up in several gun value guides. The best one is called "Blue Book of Gun Values". Your local book store probably has one that you can look at without having to buy one. Just remember to take all the important information about your gun with you when you go. Make, Model, condition, caliber, etc...
I have a Blue Book here somewhere that is a couple of years old. PM me if you want me to look it up for you.