How Do You Use Price Earnings Ratio?

When the market is getting crushed and it's a peak financial opportunity to be getting some of the best deals out there, we see cash levels spike up. What do if you were using indicator like price earnings ratio, you hear people talk about price earnings ratio? How do you use price earnings ratio to figure out where we are in this economic cycle too? I think too many people die on the hill of price to earnings ratio as the end all be all to valuation.