I won't say whether this is a bubble or not, but the number one thing I look for is businessweek (or newsweek) saying that "things have changed!" A mainstream publication putting the bull market on its cover is the prime indicator that it is a bubble and that it will be a painful crash.
It is almost a cliche.
The real bubble right now is in the dollar. We've been able to inflate, an export that inflation, for nearly 100 years, and we got away with it because of military might and manufacturing might (and bretton woods). 2 of those 3 are gone and the last one is losing respect rapidly. It is gonna be a bloodbath when those dollars come home to roost. (and it will kick off a bubble in gold.)
Maybe the bursting of this "tech bubble" (if we're in one) will be the trigger that causes the end of the dollar bubble.
Unfortunately I'm not aware of business week putting the dollar on the cover and saying "This time its different". (But maybe one of their covers for gold will count.)