[replooda]

Time, I would say. First, there's the sense that the poison will produce its effects, revealing itself in hindsight; of course, we'd rather catch it before it does, but that isn't always possible, and the best we can do is to limit the damage and apply an antidote as soon as possible. Second, time brings experience, which allows us to do that, recognize symptoms, early one and, with enough of it, enable us to do the closest thing to identifying it before it causes problems, by identifying patterns, comparing a potential skill with other skills which had turned out to be poisoned — hype can be a good indicator. I wish I could give you a set of rules by means of which you could know for sure beforehand, but I really do believe that this is a domain for heuristics, and subjective ones at that — insofar as a skill can be professionally valuable, yet dreadful at a personal level.