IMO, the best indicator for a bubble is when people participating in some financial activity look at you like you're an idiot when you ask basic questions and can't explain themselves w/o all sorts of complicated answers and rationalizations, ultimately followed with "you just don't understand."

I get that there are superpeople who are really that smart, but in my experience 99% of you are not that person.

The hard part is guessing the limits of irrationality and maximizing your profits. I got out of the dot-com bubble over a year early.