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I'm not admitting that I use to be a spy at all, but a technique in simple spycraft is to place a human hair using a tiny quantity of any substance remotely sticky onto something and then you will know whether it has been moved or not, because the hair has moved or become detached. I guess you can get this trick to work in a similar way.

I still have a set of the steel "Hindoo Cones" that were sold by Ellisdons. They work in exactly the same way that has been described above. mine came with a rather pathetic cardboard disc, but I always used a coin.

You could use the Hummer cups method with no gimmick at all.

Barry