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What Happened To Duracell PowerCheck?
(hackaday.com)
This is a most excellent place for technology news and articles.
My dad used to just put them on his tongue for a power check. Not entirely sure how that worked. Personally I just use a multimeter.
You can do that with the 9 volt batteries and feel how much "tingle" they have, but never heard of it working with aa batteries. Wouldn't he have to stick the whole thing in his mouth to complete the circuit?
I do this with AAs. Basically put the side with the bump (positive) on the tip of the tongue and touch the flat (negative) side with a wet finger. It gives out a mild but distinctive 'taste', not enough to tingle but definitely something I am able to notice, when the cell has decent juice.