S&P 500 PE Ratio is a pretty good metric to predict US Stock Market trend.

Below chart shows the ratio from as early as 1880 till date. The red dot on the right side of the chart shows the current value.

Nowadays PE ratio is way higher from what it was a 100 years back, owing to the fact that everyone's more aware today with the internet revolution. Its also because of the fact that, the choice of stocks in the index are also aggressive.

Even today, PE ratio of above 25 or 30 is not easily sustained for too long and eventually falls. So, predicting a stock market crash should not be all that difficult