The true answer? Well you could very carefully measure the sg, and then send a bottle to a lab for testing. Then determine the factor knowing the result.
Steve
I'd love to have another toy - an ebulliometer. I recently visited with Daniel Pambianchi who owns Maleta Winery (also the author of Techniques in Home Winemaking - a book I highly recommend). He offers lab analysis of wine, including determining the alcohol content of a wine.
To do this, one uses the ebulliometer after it has been calibrated for atmospheric conditions (boiling point of water is slightly different depending on the atmosphere pressure), and then the wine is boiled (alcohol has a lower boiling point than water). A very high precision thermometer is used - it's about 12 inches long, I think.
Once the boiling temperature of the wine is known, you can determine the percent alcohol.
In Ontario, I believe commercial wines are allowed a 1% variance in the listed alcohol percent on their labels.