If you really want to know: purchase a heat-resistant 100 ml graduate cylinder and a good thermometer. Place EXACTLY 100 ml of ale in the cylinder (noting original temperature) and heat in a water bath until the ale reaches 78C. At this point, the alcohol will start to boil off and the temperature will stay at 78C until almost all of the alcohol is gone. When the temperature starts rising again, cool the cylinder to the original ale temperature (so the water shrinks to its original volume). Read the level and subtract from 100. That's your ABV.
If you have a really good scale, you can measure the weight change.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
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