The evidence is found in the corporate balance sheets, income statements, P/E ratios, dividend yields, and other quantitative measures. It is likewise reflected in low ratios for the market as a whole. The quantitative factors speak for themselves.
The dividend yield on the Dow Jones Industrial Average, for example, usually cycles between a high yield of 6 percent at the market's bottom and a low yield of 3 percent at the top. The Dow's average dividend yield sometimes stretches beyond these boundaries, but historically this is a trustworthy parameter of undervalue and overvalue.