They can't. Senator Doctor Tom Coburn has nevertheless offered a pathetic, idiotic diagnosis that Bowe Bergdahl was drugged based upon the good Senator Doctor's observations from the so-called "proof of life" video. But, there is a good reason that, in court, proof that someone is on a drug requires a properly conducted laboratory test, of a legally obtained, secured and preserved tissue sample. Strict proof is needed, because, with exceptions for extreme doses and occasional manifestations like pupil dialation/contraction, use of many very powerful drugs often cannot be detected by physical examination by a physician, much less by watching a brief recording made under non-clinical conditions. Where there are physical manifestations, physical examination usually offers little to differentiate among causes.
The story of my Armed Forces Induction Physical in 1971 illustrates the point. I was tripping on LSD at the time, and verily baked on some nice bud, as well. Though I was examined that day, every which way, by numberless physicians, nurses and other medical personnel, nobody noticed a thing. Go out into the tall grass if you are interested in the story of that day.
I enlisted in the US Navy in 1971, facing a low draft lottery number and the expiration of my last deferment. I did not wish to be at the mercy of Selective Service regarding the selection of a branch of service in which I would serve and also wanted to have some control, if possible, over where I would receive basic training. I was reporting in Winter and wanted to avoid training in the upper Midwest, preferring Southern California at that time of year. The answer to that was to find a Western city that sent its Navy recruits to California and enlist there.
So it happened that I found myself 400 miles from home, getting ready to report for my induction physical and immediate transportation to Navy boot camp, in the company of my best friend from college. A drummer. A really great drummer.
We had toured together for years, tasting much of the best that the Sixties offered. My wife couldn't stay that penultimate day and night because of her job. So, my college friend took it upon himself to make sure that I stayed in a good frame of mind as I faced my onrushing doom, during the final 36 hours.
He had never given me bad advice about how to accomplish such a thing, and he did not this time, either. I won't try to describe those mostly sleepless hours except to say the experience was rather sublime. Understandably, most of the memories, per se, are non-episodic, fragmentary and semi-coherent at best. And there, of course, lies the explanation for the load of LSD and THC I was packing when the medical folks got to me the next morning. Nobody noticed anything.
I finished boot camp being awarded with the highest decoration given to a graduating recruit, the American Spirit Honor Medal. Go figure.