It's this weird thing called science. Alcohol and water are miscible, and when together they azeotrope. This mean that they boil off together, at a different boiling temperature than either of the original substances, which can be intermediate between the two, or in fact lower than both. In this case, ethanol boils at 78.5C, water at 100C, and the two azeotrope at 78.2C. In addition, the gas phase from the azeotrope contains 95.4 percent ethanol, and 4.6 water. This is the reason ethanol cannot be distilled to a higher percentage than 95% (that is, unless you then azeotrope off that water using benzene, which ultimately leaves trace benzene, an aggressive carcinogen in the remaining absolute ethanol). What this means of course, is that the boiling point of ANY water/ethanol solution is 78.2C. So since the temperature of this solution cannot rise about 79C until all of the ethanol is gone, this is an easy way to know. Of course, the caveat to all of this is that these boiling points will all be slightly elevated because of the dissolved small molecules (residual sugars, etc) that make the water/ethanol mixture beer.