Irrational Exuberance, by Yale economist Robert J. Shiller, details historical indicators of stock market bubbles. The following is a list of these indicators.
1. High consumer confidence. (from sources such as the Consumer Confidence Index and the Michigan Consumer Sentiment Index) For example, both indexes peak during the 2000 bubble.
2. High PE ratios.

When the market reaches PE ratio extremes on the high or low end there is a reversion to the mean. The pattern of market bubbles to be linked with high PE ratios is illustrated by the first graph above. …