The Q-Ratio, created by James Tobin, is a very good measure of valuation:
What Does Q Ratio (Tobin's Q Ratio) Mean?
A ratio devised by James Tobin of Yale University, Nobel laureate in economics, who hypothesized that the combined market value of all the companies on the stock market should be about equal to their replacement costs. The Q ratio is calculated as the market value of a company divided by the replacement value of the firm's assets
Investopedia explains Q Ratio (Tobin's Q Ratio)
For example, a low Q (between 0 and 1) means that the cost to replace a firm's assets is greater than the value of its stock. This implies that the stock is undervalued. Conversely, a high Q (greater than 1) implies that a firm's stock is more expensive than the replacement cost of its assets, which implies that the stock is overvalued. This measure of stock valuation is the driving factor behind investment decisions in Tobin's model
Doug Short is publishing a nice chart of the Q-Ratio which shows how extreme the valuation is now becoming:
click to zoom
As you can see, the Q-Ratio is now reaching extreme values, quite historical ones actually, as it is now above the level it had reached at the very top of the biggest bull/bubble markets of the past 100 years: it's now higher than the top of 1929 and of 2007, second only to the tech bubble of the early 2000s.