Take a hydrometer reading right before you pitch the yeast into your wort (see "How do I take a hydrometer reading?" in FAQs). Make a note of this reading, which will vary by style but should be larger than 1.040.
After the fermentation is complete, take another hydrometer reading right before you bottle. Be sure you take your reading BEFORE you add the priming sugar, as this will skew the reading. This reading will usually fall between 1.010 and 1.030, again depending on the recipe and style. Then simply subtract the second reading from the first and multiply by 131. This will give you a very close approximation of your alcohol by volume.
Example:your starting gravity reading is 1.055 and your final gravity reading is 1.012 the math would look like this:
(1.055 - 1.012) * 131= 5.6
therefore your alcohol by volume would be approximately 5.6%.