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I'm curious if there is a basic set of tests you can do with drinking locally to know one way or the other, as well?

Measure your blood pressure.

Measure before consumption. Take three different readings.

Measure at one hour intervals after caffeine consumption.

Fast metabolizers BP should return to baseline within a couple of hours.

Mine would remain elevated for 10-12 hours.

Did you measure your baseline blood pressure without caffeine as well?

Yes.

How many hours/days/weeks should you be caffeine free for the baseline BP?

A couple of days.

I usually have to go through a withdrawal headache first.

I'm assuming elevated is purely relative to your baseline?

Oddly, I don't think my blood pressure changes much at all. Will try this test again, though.