Less than two years after the housing market collapsed, the U.S. economy is threatened by a new bubble in asset prices. This time, four billowing balloons are hovering: two commodities -- gold and oil -- stocks, and government bonds.

Don't be fooled into thinking that last week's 5% drop in the S&P, and the recent sell-off in oil, remotely makes them fairly valued, let alone bargains. Equities and commodities, as well as Treasuries, which actually rallied as stocks dropped, still have a long way to fall. The reason: They've already seen huge run-ups that put their prices far above their historic averages, and far above the levels justified by fundamentals.

Two examples: Most companies can't possibly grow earnings fast enough to support their lofty valuations, and oil and gold are so expensive that we'll see what high prices always bring, a surge in new supply. That makes a price-pounding glut inevitable.

Since the start of 2009, oil has returned to the danger zone by jumping 63% to $75 a barrel, and gold has risen more than 20% to set astounding new records by climbing above $1,100 an ounce. After briefly returning to historically normal valuations in March, stocks are now selling at price-to-earnings multiples 40% above their historic range of 14, and 10-year Treasuries are so pricey that they yield 1.5% less than they did in 2007.

What's causing this resurgence of speculative fervor? One view blames the same policy that caused the real estate rampage -- incredibly low interest rates that are flooding the banks with cheap funds that, in theory, are available for loans. (The current Fed target rate is between 0 and 0.25%.)

"Investors can borrow at extremely low rates to buy assets," says Brian Wesbury, a monetary specialist at mutual fund manager First Trust. "So they're using cheap debt to bid up prices. The Fed's expansionary policies are making assets look a lot less risky than they really are."

Other prominent economists dispute that we're in bubble territory, at least right now. Allan Meltzer, the distinguished monetarist at Carnegie Mellon, argues that even though banks are loaded with cheap money, they aren't lending -- which is why we have a credit crunch. "I would be a lot more concerned if loan demand were higher," says Meltzer.

The one asset that definitely isn't bubbling is housing. There, prices have fallen to a level where new buyers buy a house for the same total monthly cost as rental. That's gravity operating.

So how do you spot a bubble? My view is that we're now seeing the same signs that exposed the frenzy in real estate: prices flying far above their historic averages, measured either in inflation-adjusted dollars (commodities) or as a ratio of the income they produce (stocks and Treasuries). Watch for gravity to take over, just as it did in housing.