Warren Buffett once said that the percentage of total market cap (TMC) to the United States GNP is probably the best measure of where valuations stand at any given moment.
In the past forty years U.S Stock Market history, the TMC to GNP ratio has varied within a very wide range. The ratio was lowest at 0.35 times in the previous deep recession of 1982, while the highest point was 1.48 during the tech bubble in 2000. As a rule of thumb we can broadly divide the market valuation in the five bands based on past records:
Ratio < 0.5 - Significantly Undervalued
0.5 < Ratio < 0.75 - Modestly Undervalued
0.75 < Ratio < 0.90 - Fair Valued
0.90 < Ratio < 1.15 - Modestly Overvalued
Ratio > 1.15 - Significantly Overvalued
Check out the current and historical chart of this ratio from here: Market Cap to GDP Ratio