The standard methods that the ATO accepts are Distillation and Near IR Spectroscopy.
Distillation isn't all that hard and a lot of the larger wineries use this method, NIRS on the other hand needs some pretty expensive kit ($20K+) so would probably only be found in really big wine or brewing companies.
If you had any wineries around where you live it might be worth making some phone calls.
I would need to do a bit more digging but I think if you can take both a Hydrometer SG and a Refractometer reading you should be able to interpolate the answer, maybe not well enough to keep the excise man happy but close enough for home brew. (
have a look at this)
Otherwise if you took your OG and FG readings and calculated your alcohol in the normal way. Having a look at your current SG, if its gone back up, you should be able to make a fair guess as to how much alcohol you have lost. in Scotland, in a cellar (so stored around 10oC) whiskey is said to loose about 4% of its alcohol/annum (the so called "Angles Share").
Here its hotter but your alcohol is lower than cask strength spirit and a lot less volatile, so I wouldn't be too worried, there will be plenty left.
Mark