Tests exist for fentanyl, a quick google shows at
least one lab offering the service, from urine specimens via GC/MS of metabolites unique to fentanyl, not to mention the not inconsiderable amount excreted unchanged. I think they meant that it is not readily obtained by dipstick style immunoassay.
That being said: Newsflash, all sorts of people in life critical settings do drugs. What you would really need to show here is that of all people in the world, an anesthesiologist not being capable of judging a dosage for him or herself that would allow them to carry on their function safely. In any event, I would judge the physicians performance, not his substance use. It seemed to offer one or two story of dysfunctional hacks, albeit with no convincing evidence that he was such only by reason of his drug use. Along with this "the anesthesiologists most at risk of addiction are the best in the business. They tend to be young, ambitious, highly talented physicians who graduated at the top of their medical-school classes and are ranked among the most popular and requested at their hospitals."
Tl;dr it sounds like media hype.