Answer:
You could perform a study on animals or humans to show an effect of caffeine on the kidneys - in a controlled experiment you would look at renal function (such as creatinine, creatinine clearance, BUN), renal histopathology and even metabolism.
Many caffeine studies have already been done and I am currently running one right now - the affect of our drug on the CYP 1A2 enzyme responsible for caffeine metabolism.
checking things like bun and creatinine and urine output are a good start, but there may be other things worth checking. for instance, what does it do to renal blood flow? modest increases in renal artery blood flow may not necessarily show up in blood tests like bun/creatinine. an angiogram to directly look at blood flow might be useful.