I read a post, and find that people are looking at the trend of ^VIX .
^VIX is probably an important indicator.
Here is result of searching ^VIX definition from the web:
^VIX shows the market's expectation of 30-day volatility. This volatility is meant to be forward looking and is calculated from both calls and puts. The VIX is a widely used measure of market risk and is often referred to as the "investor fear gauge".
