Has anyone ever reliably made profits from bubbles? If so, their existence can be prospectively determined, if not there is no clear answer. It's true that there may be people who know a bubble with 100% certainty, but they don't know where they are in the cycle so they are too worried about shorting them. This is pretty morally equivalent to having no idea about the existence of a bubble, although not quite. I'd say if someone can reliably predict that within a "reasonable" time the bubble will deflate below the current level, and are willing to bet on that, they "know" the bubble exists.