How to tell if we are in a bubble

Groupon may have shelved its IPO, but with Zynga still looking to raise a billion and LinkedIn trading for almost double its IPO price there is once again talk of a tech bubble. Sure, the traffic is heavy again on US 101, there are parties every night in Silicon Valley and companies are building tree houses in the headquarters, but how do we know if this is the real thing?

Here are some of the more interesting ways to tell:

• When companies invent new accounting metrics that ignore inconvenient costs. In the dot.com bubble we had proxy valuations, recently we had Groupon trying to push ACOSI as an alternative that ignored the cost of acquiring customers.
• When founders take out vast quantities of cash while a company is still losing money.
• When newly minted MBAs flocking to the field. Marc Andreessen recently argued that during the dot.com bubble MBAs flooded into tech companies for the parties and the hype. Then they all went to Wall Street in time for the most recent financial crisis. He says that hasn't happened yet in tech.
• When corporations get into "strategic investing" which doesn't require that the ventures they invest in can turn a profit.
• When Angel Boot Camps spring up like mushrooms.

But I think the best indication that we may be in a bubble is when everyone says we are not.