I'd assume it's done through DNA-tests to avoid slip-ups, after all, if there can be one, there can be more. After all, not all powers are easy to spot on their own, looking at my own (powered) character, the only surface-feature is that her heart can beat really fast without risking cardiac arrest. If all that mattered was surface traits, then a significant amount of people at Denai would not be branded as abnormal despite being so. In theory, a slip up would be possible, but between visible signs and tests, the odds are rather microscopical. Theoretically, it'd be doable, but practically, not very likely.