Ed Hyman, legendary economist for Evercore ISI, included a chart in a note to clients last week of Sotheby's stock price over the last three decades.
When shares of the art auction house climbed to new heights, it marked the top in the Japanese financial bubble of the late 1980s, the technology stock market bubble of the late 1990s and the housing market bubble at the start of the Great Recession.
"Surges in Sotheby's stock price have had an uncanny correlation with bubbles," Hyman wrote under the chart titled "Art as a Bubble Barometer." "The stock is flat to down over the last three years."
Hyman is implying U.S. stocks are not in a bubble this time despite a tripling in the value of the over the last five years to a new record.
The economist and other strategists reason that when investors accumulate too much wealth by way of a financial asset bubble they spend it extravagantly on pieces of art, lifting Sotheby's stock to overvalued levels.
That hasn't happened this time. Sotheby's last month reported a wider-than-expected loss for the third quarter as private art sales—just the kind that would get hedge fund honchos and oligarchs excited during past bubbles—disappointed.
Also in Hyman's favor is the fact that the Nasdaq Composite is still below levels it reached during the height of that tech stock bubble in March 2000.
"I won't believe it's a (stock market) bubble until average investors begin levering up and buying stocks," said Jim Iuorio of TJM Institutional Services in Chicago. "This has not happened yet, so my belief is that the market rally has more room to run."
In China, officials are going out of their way to tamper a bubble. Securities regulators are increasing their checks of brokerage houses to make sure they are complying with margin trading rules, actions that weighed on Chinese stocks last week.
To be sure, the bears have already labeled this bull market the "Fed Bubble." They argue that when the Federal Reserve begins raising interest rates from zero (likely in 2015), stock prices will give back much of their gains since the credit crisis.