Have the subject write down a random number from 1 to 1024. Perhaps this random number is assigned via a phone app. Have the subject put the paper in his pocket.
An maximally accurate polygraph will require no more than 10 questions to determine the number in the subject's pocket.
[However, I think we all know what the result would be, which is why nobody publishes a simple calibration like the above: It is not in an trained examiner's interest to expose the inaccuracy of polygraphs.]
Which begs the issue: If a lie detector is accurate only for answers that are stressful, then it sure looks like we need a detector to figure out if an answer is stressful or not.
Or to put it differently, now you have two problems: Lie detection and stressful-answer detection.
Nevertheless, I concede that your beer detection plan is superior.
Therefore I too must concede that the beer plan is flawless.