A few macro links to ponder on the subject of asset price bubbles:
Are we in one?
No, we aren't, says MIT's Ricardo Caballero; asset prices are up, but it's not a bubble, it's a rational response to a shortage of assets in the face of high global demand. First Japanese assets melted down in the early 1990s; then Europe stagnated in the late 1990s; now China and the oil states are evincing huge demand for assets but aren't producing many themselves. Bubbles are, in a way, just the market's response to a shortage of assets, and trying to control them could cause further damage.
Former US Treasury economist Thomas Palley disagrees, in a response to Caballero: the last couple of decades have actually seen "an explosion in asset supply" thanks to privatisation in the advanced economies of the West, government deficit accounting and now privatisation in China as well. Instead, Palley wheels out eight other demand-led explanations. Rising inequality means that the rich (who tend to invest) have a great deal more money, especially in the US; higher corporate profits and lower corporate taxes mean that equities are fundamentally more valuable; he also cites lower interest rates, innovations in the markets and "plain old mania". Bubbles, in this interpretation, are dangerous; both in themselves, and as a sign of underlying imbalances and inequalities.
In any case, could we tell if we were in a bubble?
No, according to a Federal Reserve working paper from Refet Gurkaynak: there's no accurate econometric way of detecting bubbles, and whether an economist decides that a particular scenario is a bubble or not is "a matter of taste and personal preference".
Though, apparently, your chances of detecting a bubble go up with age: Robin Greenwood at Harvard and Stefan Nagel at Stanford note that young fund managers tended to pile into the late-90s tech bubble, and got their fingers burned when it burst in 2000. Nagel's found the same thing to be true at the individual level - people who grew up during bear markets tend to be far less heavily invested in equities.
"Go out 20 or 30 years from now", Nagel says, "and it's possible we'll see another bubble" - led by fund managers too young to remember the 1990s, and too confident to believe their grizzled elders' horror stories about AOL Time Warner and Pets.com.