You can estimate the ABV pretty close by knowing your starting and ending specific gravity from the hydrometer - both of which must be corrected for temperature. Probably won't work for you this time since the starting SG is not known but will be helpfull for future batches.

Calculate the percent ABV by subtracting your adjusted final specific gravity from your original specific gravity, multiplying by 1000, then dividing by 7.36.

For example, if original specific gravity is 1.095 and final specific gravity is 1.008, multiply the difference (0.087) by 1000 (870) then divide by 7.36 to get an ABV of 11.8 percent.

Hope this helps.