Well, that's kind of what you have to do. The definition of a bubble is contingent on the burst at the end. If it never bursts, it wasn't a bubble. So you take

Well, that's kind of what you have to do. The definition of a bubble is contingent on the burst at the end. If it never bursts, it wasn't a bubble. So you take today's valuations (which you assume are bubble-inflated) and then compare them to the valuation at some future date. If the valuations are a lot lower, then there was a burst and thus a bubble. If not, no bubble (or it's still ongoing).