The one that I look for, which I derived from reading Michael Lewis's Liar's Poker, and from my experiences in the CD-ROM publishing business in the mid 90's is this simple rule:
When expensively educated, fashionable young graduates start showing up in your field, you're in a bubble.
Lewis describes this in the investment banking business, when his entire Yale graduating class applied to become investment bankers.
The trouble with this indicator is that if you aren't looking for it it seem like the natural order of things - of course having personable young things hanging on your every word is to be expected - finally you're getting the recognition you deserve!
In practice, however, the finely-tuned herd instincts that get selected for in the Ivy League or the posher UK universities make them flock to the latest bubble.