In Security Analysis, Ben Graham and David Dodd advocated averaging earnings over several years to gauge a company's earnings power, and from that idea comes the concept of "PE 10", which is the P/E of the market using the current price in the numerator, and the average of the last 10 years of earnings for the index in the denominator.
Since 1880, the median and average P/E 10's of the S&P 500 have each been around 16. Currently, the S&P 500 trades at about 19 times its earnings over the last 10 years, which is well below the 44 times it reached in 1999.
One site, multpl.com, is dedicated to providing a daily update of the market's P/E 10. Readers are encouraged to visit this site from time to time when they wish to determine the relative cheapness of the broad market, as it can serve as a useful anchor amidst the media noise that attempts to predict market movements using all sorts of various, often irrelevant, data.