Well, that's the thing about a bubble: it can be very possible to accurately tell it's a bubble—and the definition of a bubble is that it's going to pop—without being able to tell just when it will pop.
And given how much of stock prices these days is purely dependent on stock traders believing they'll be able to sell those stocks on to some Greater Fool later, rather than based on the reality of the company the stock is about...well, they can keep the ball in the air for quite a while.
Personally, I suspect that the primary trigger for the big selloff is going to be the upcoming IPOs. Bigwigs get their big bucks, cash out, and leave everyone else holding the bag.
But if any of us knew when events like those were going to happen, we'd probably be rich from that already, hm?