This was super interesting! It's disappointing that it takes a $2700 machine and $10 in consumables per test to learn this, but I'm glad we finally have access to this kind of information.
It's clear that caffeine is detected through a colorimetric reaction (the murexide reaction), consequently old-school hackers will use a Spec 20: https://en.wikipedia.org/wiki/Spectronic_20
2700 dollars for a single-wavelength colorimeter? That's madness!