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There are a lot of questions here looking for simple methods to measure caffeine content (or extract it), including one of mine here and on the Coffee site:

But here I'd like to ask something different:

Question: Is there a standard industrial method for measuring caffeine content in food and drinks?

For example, if there were government requirements to post the amount of caffeine in a product there may be some standard way that government advised that it be done (i.e., "this is the way we're going to do it to verify what you report."). While I don't know of any such requirements, that doesn't mean there aren't any.

uhoh
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The answer is fortunately rather simple. Use a C18 reversed phase column and a UV detector in an HPLC system. One would make a calibration curve by injecting various concentrations into the column. Now, if an unknown sample of a drink is injected, and one sees a peak at the same retention time in the sample chromatogram, you know it is caffeine. By interpolating the area, we know its concentration. When an HPLC method is developed, it is ensured that no other component elutes at the same time. This is achieved by using gradients of mobile phase composition.

If there is a legal issue and one has to provide a 100% foolproof test, then one has to use a mass spectrometer along with a UV detector to confirm the identity of the peak at various locations, i.e., one ensures that the peak is pure.

See the examples in Agilent's note "Determination of Caffeine in Coffee Products According to DIN 20481 by Edgar Naegele from where this figure is taken:

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AChem
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    I could see series caffeine aficionados laying out $50k for an HPLC. How else do you tune in the espresso machine for that perfect cup? – Jon Custer Nov 18 '22 at 03:16
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    Determination of Caffeine in Coffee Products According to DIN 20481 (also here) compare figures 4 & 5. Nice! – uhoh Nov 18 '22 at 11:14
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    "DIN"="Deutsches Institut für Normung" - the German ISO member. DIN norms are pretty much the requested "standard industrial method". – MSalters Nov 18 '22 at 14:07
  • @JonCuster or maybe a group of them can start a local neighborhood HPLC co-op? – uhoh Nov 18 '22 at 14:08
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    @uhoh, Mini portable and complete HPLCs of the size of a brief case are available now. I see no problem in running caffeine stds and samples. – AChem Nov 18 '22 at 16:48
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    @AChem that's great! I'm afraid that the compactness in volume won't translate to compactness in cost - buying one may require a brief case full of Starbucks gift cards or equivalent. So the cost sharing motivation for the co-op idea can be translated to a traveling "coffee inspector" who makes their rounds on a weekly basis on a motorcycle using an Uber Eats or Food Panda on-demand model. I like it! I'm gonna be rich! – uhoh Nov 18 '22 at 22:06
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    @uhoh We used to keep a 5 pound bottle of caffeine citrate next to the always on lab break room coffee pot. The sign on it, due to me, was “Two scoops per pot, to combat the Santa menace.” So you could just start with decaf and add caf, to desired level of wiredness. (We took the big bottle of caffeine citrate away after a few weeks, because someone might have not understood and actually added the two scoops!) – Ed V Nov 18 '22 at 23:40
  • @EdV Not a chemist can you explain lol – Azor Ahai -him- Nov 19 '22 at 22:54
  • @AzorAhai-him- My comment had a typo: “Santa” should be “Sanka”, a brand of decaf coffee. In a large research group, coffee “powers” progress and decaf would be pretty much useless if you want to facilitate productivity. Our group, run by a famous chemist (not me!), typically had around 25 grad students, around 8 post-docs/sabbatical leave folks, and we had undergrads doing short research projects, though we never made a list of them because they came and went semester by semester. So the caffeine citrate was just kind of a joke, with a little point: get stuff done! – Ed V Nov 19 '22 at 23:18
  • @EdV Why can't you add the two scoops? – Azor Ahai -him- Nov 19 '22 at 23:24
  • @AzorAhai-him- Two scoops would be way too much and could have very bad medical consequences! – Ed V Nov 19 '22 at 23:26
  • @EdV Ah, lol. . – Azor Ahai -him- Nov 19 '22 at 23:27
  • for non-chemists like myself this video and lab writeup titled "Lab 7: Caffeine Quantification by HPLC" are quite helpful. – uhoh Dec 02 '22 at 22:42