What was the Starting Gravity/Original Gravity of your wort, before you pitched yeast?? how long has it been fermenting??
If it's an extract brew and you didn't measure the SG/OG you will be able to roughly calculate that figure based on weights of fermentables used and volume of wort into the fermenter.
after fermentation has wound down, as many of the available, fermentable, sugars that can be converted into alcohol and CO2 by the yeast have been, you'll be able to read the Final Gravity, FG.
your Alcohol By Volume %age can be approximately calculated by
(OG - FG) X 131 = ABV%
for example (1.048 - 1.012) X 131 = 4.7%