Asking a VC to determine whether or not there is a VC bubble is like asking a mortgage broker or real estate agent whether or not there was a housing bubble during 2006. They have a self-interest to believe that the good times will keep going. During the dot-com bubble and the housing bubble, the rationalizations that were being spouted by those in the midst of it were incredible.
The same goes for now. There is no metric by which Whatsapp is worth 19B, except for the fact that Facebook can spend that much money. Any attempt to monetize those users will result in decreased users.
The only thing keeping the valuations high are because people delude themselves into believing that Google or Facebook will pay billions for customers. If Google and/or Facebook declared they would be doing no more acquisitions, valuations would plummet immediately.
The valuations given to companies with no real revenues, or profits to justify ridiculous valuations. But the rationalizations that get spouted to justify them are what is a big indicator of a bubble to me.