A coin dealer will always be able to tell you the value of coins and old paper money, but you should be able to do some of the work on your own. Online and print sources chart the going prices for older money, and what makes it worthwhile.
Check auction web sites such as Heritage Auctions. Check the money you have against money put in recent auctions, or post the bill and see what site officials or users say about its worth.
Determine the condition or grade of the money. The best grade, "Superb Gem Crisp Uncirculated," is for bills with no folds, no bends and perfect centering. The grade goes down to "Good," which covers a bill that is heavily worn, taped and may even have tears in it. A "Good" grade means the bill is in bad shape but is still intact enough so people can see what it is.
Check web sites devoted to numismatists and learn what bills are considered the most valuable. As of 2011, some valuable bills included a 1907 legal $5 bill with the image of a farmer's family and any genuine Confederate currency. Depending on the condition, the 1907 bill could be worth between $80 and $200, while Confederate money from 1864 could run between $200 and $400. A Confederate bill from the first year of the war, 1861, is worth much more.