A beer's alcohol content is determined by measuring the original gravity (OG) prior to fermentation then the final gravity after fermentation.
ABV = (OG - FG) * 131.25
It is important to note that the above formula only yields an approximatation of alcohol by volume (ABV). Realistically speaking, the formula lessens in precision as the alcohol levels increase. If you do not already possess an accurate alcohol by weight measurement, an alternative formula includes the following:
ABV = [ABW * (FG / .794)]