Probably the easiest way is by using a hydrometer. Most of the above-profiled home brew kits supply one and they’re pretty easy to use. The hydrometer measures what is called the “gravity” of the brew. Gravity is a measure of the density of a given liquid when compared to the density of water. As the yeast in the fermenting brew gradually consumes the sugars the brew itself becomes less dense. To obtain the alcohol content of your brew use the hydrometer to measure the gravity at the beginning of the fermentation process (called the Original Gravity or OG) and then measure it again at the end of the process (called the Final Gravity or FG). Then plug both numbers into this equation to get the alcohol content:  (FG - OG) x 131.25 = ABV %.