Might be a duplicate here: http://homebrew.stackexchange.com/questions/10823/how-to-calculate-alcohol-without-og-reading

It appears that there is a formula using your Specific Gravity from a hydrometer and if you pick up a refractometer, a measurement from that, that you can use to calculate the ABV. 
Jump to section "Measurement of ABV" at [Brew Your Own](http://byo.com/stories/item/1313-refractometers) for the formula.

From the article:
> ABV = [277.8851 - 277.4(SG) + 0.9956(Brix) + 0.00523(Brix2) + 0.000013(Brix3)] x (SG/0.79)

> In this equation, Brix is the Brix reading of your refractometer and SG is the specific gravity reading from your hydrometer.

> If you try this, it is important to take very careful readings. Degas the sample in a blender or by pouring it between two glasses until it does not foam. Make your reading at as close to 68 °F (20 °C) as possible. If your hydrometer is calibrated in Brix, use the longer formula I cited earlier to convert it to SG. This equation fits very well with the data points. Measure-ment of ABV can be made to within 0.3% if you are careful.