Someone posted a thread here on exactly how to do this. I tried searching for it but I couldn't guess the right keywords.
It was a bit technical for my knowledge but basically he said you take a specific gravity reading. You the then boil it some and then top it back up with water to the original volume. and measure the specific gravity of that. He said *viola* that's your o.g. although I think he meant the difference between the two can determine your o.g.
*My* take is: If you boil you remove the alcohol but keep your sugars. Hence if you compare the two potential alcohol reading the difference should be your current actual alcohol. ..... shouldn't it?.....
And then if you still want the O.G. (why? you have the ABV, so....) You can take the difference and add them to you current specific gravity (well, I don't know if the gravity readings are linear but you could add your ABV to you potential alcohol to get your original potential alcohol and figure your o.g. is the gravity that corresponds to that.)
Now, can somebody who actually knows his ass from his elbow verify and expound on that? (or find the other thread)