I know that the decaffeination process is not perfect and that it leaves some caffeine. I've looked online and there was a company in 2009 that offered caffeine test strips but they seem to be no longer offered.
Another approach would be to use a GCMS, but used GCMS devices are expensive and it takes alot of effort to keep them in calibration (at least it did when I was in college).
Are there services where I can send a sample off and they will tell me how much caffeine they contain? Failing that, how would I reasonably test how much caffeine is in my tea and coffee?