A decaf tester?

What we need is a thermometer style stick to test coffee for absence of caffeine.

Today I bought a small decaf latte from a caterer which specifically advertised a decaf option. This was welcome - I'm not sure whether the availability of decaf is improving or whether I'm just more discerning. For instance at a motorway service station I will head for Costa or Starbucks and take a decaf into the restaurant. No point in asking there as the best I will get is a dreadfully made instant sachet. I do drink a lot of instant coffee but it needs milk.

As I came away my throat was choking and that left eye started to ache - a surefire sign that something is wrong. Apart from that I felt bright and cheerful. It was a pleasant day - and I felt quite high ... as you would after an unexpected shot of caffeine. One possibility - as I have noted before - is simply that filter decaf or whatever they use in coffee shops is not fully decaffed. Another is that staff don't always remember to get out a decaf sachet and think you won't notice.

So we need a discreet testing device. Can it be done? A device for testing for salicylate would also be handy - but a little more complex as it's not going to be found uniformly in any food. So it would have to be some kind of scanner - maybe a project for the 22nd century?

RAS