The potential ABV scale on the hydrometer is to give you an idea of your potential based on initial gravity (but it really only works for wine) - but if you subtract your final gravity from your initial gravity and multiply the results by 131, you'll have your actual ABV (assuming your hydrometer is correct and you corrected for temperature).
That ones a real figure, not a "potential". You can't get the real ABV of course until it's done. And once it's done the potential scale means nothing.
if you have a chem lab i would think taking a sample. measure its volume. burn off as much of the the alcohol as possible then measure volume again. though likely not as accurate as a good og-fg calcuation.
probably other more sophisticated ways but this sounds pretty simple if you dont have a good og/fg
The hydrometer is simply measuring the density of the sample. If you have a good scale and a way to measure the volume accurately, you can do the same thing as a hydrometer.
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The simple, cheap methods are no more accurate than a hydrometer. To get an accurate value, you'll need a good graduated cylinder or a precision scale and a way to boil off the alcohol without losing water.
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Like the other said, take a gravity reading before fementation (original gravity or OG), then you go back and measure it AFTER it is done fermenting (final/finished gravity or FG). Make sure you keep note of the temperature of the wort/beer at time of the reading. Plug the numbers into a formula, beersmith or the many calculators on the internet.
That will give you a very accurate measure of ABV. That number is not an estimate, since the formula is calculating the fact that as sugars get converted to alcohol, the density of the sample will drop. They have figured out via calculations how much alcohol that drop in density corresponds to %alcohol.
Hope that was helpful. Don't use the potential ABV on your hydrometer. If you want a good guesstimate, get beersmith. It will tell you potential alcohol based on its calculation of what your OG and FG SHOULD be.
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Any method to actually determine ABV %...
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