There are plenty of savings bond value calculators available on the internet, but you can just download a pdf of all the redemption tables from the U.S. Treasury. To find the current value of your savings bond, find the correct series and the date of issue. This link shows bonds issued from 1941 to 2011, and displays their value if they were to be redeemed in April 2012. Find the value of your bond along the top row, marked "Redemp.Value." Cross-reference it to the month and year of your bond's issuance. The amount there is how much of the face value you'll receive upon redemption (these bonds are all less than face value because they haven't matured yet, but you can also find the value of matured bonds at the link). The next column over shows the interest earned on the bond. Add the two numbers together to determine the total amount you'll receive upon redemption.
Finally, look at the very last column, which shows percentage yield. You can see that fluctuations in interest rates affect the percentage from year to year, but overall you earn more from a bond the longer you hold it, up to 20 years, when it matures and no longer gains value or interest.
In the next section, we'll show you what you need to redeem a savings bond.