Well, kind of a necro thread but to answer the original question, I'd think you could approximate it by knowing ABV, total volume before, and total volume after. You can calculate volume of alcohol, and if you assume you remove only water (which isn't actually the case, you'll remove other stuff and some alcohol as well, hence why I say "approximate") you can then convert that back to ABV. Say 5 gallons of 7% cider, that's 0.35 gallons of ethanol. If you remove 1 gallon of water, the 0.35 gal ethanol then becomes 8.75%.
There's data out there on how much water will freeze out at various temps but I have no direct experience. But if you don't have the equipment that AJ DeLange has (via the TMF link above), that's the route that I would go.
Souring: '14 Brett C Old Ale, '15 Lambic, '14 Lambic, '14 Flemish Red, '15 Flemish Pale, '15 Oud Bruin, '15 Session Kriek
Cellar: '10 Brett B Tripel, '11 Lambic, '13 Brett C English Barleywine, '13 Quadrupel, '13 Sour Stout, '14 Brett C Bitter, '14 Spontaneously Fermented Cider, '15 Wee Heavy, '15 100% Brett B Red, '15 100% Brett L Kriek, '15 Bière de Garde