A Numbers Game
One of the things about home winemaking that eventually occurs to everyone who does it is that you're actually making alcohol--that's kind of the point. Unlike the bottles you get from commercial vineyards and liquor stores, the wine you make won't come with a label attesting to the ABV.
Kit manufacturers sometimes hint at an estimated ABV in some of their brochures, but a hard number is something you just won't find, because it's going to be different, however slightly, for every batch.
The variation is due to small differences in sugar content, caused by variances in the kit fill itself, or in an over- or under-addition of water to rehydrate your must. This might mean as much as a half-percent difference either way, so kit manufacturers are wary about committing to a number they could be held to.
Not to worry: you can figure out the alcohol content of your wine very easily, provided you have the foresight to follow the instructions and take (and record) a specific gravity reading at the beginning of the process.
Getting an accurate reading at the beginning gives you a number representing the sugar content of the wine (yes, gravity is actually the density of the liquid compared to the density of pure water, but for our purposes we're dealing with sugar content).
As fermentation goes on the yeast eats the sugars in the juice, converting them into alcohol. As the sugar is converted the density of the wine decreases, until if finally goes below 1.000 and coasts to a stop. It's when the wine is fully fermented that we can learn the alcohol content to a pretty accurate degree.
To get the alcohol content by volume, subtract the starting gravity from the finishing gravity. Let's use the example of a wine that starts out at 1.090 SG and finishes at 0.995 SG.
0.995 - 1.095 = 0.10
Now that we have the difference between the two gravity readings, we multiply it by 131 to get our ABV reading.
0.10 x 131 = 13.1, or 13.1% ABV
Fast and easy, and accurate enough to put on your label--as long as you remember to take and record your specific gravity readings!