Market bubbles are not obvious when they are occurring. Even former U.S. Federal Reserve Chairman Alan Greenspan says he's unable to tell for sure when exuberance becomes irrational. While being questioned before Congress a couple of years ago about the 2000 tech-stock meltdown, Greenspan famously said: "As events evolved, we recognized that, despite our suspicions, it was very difficult to definitively identify a bubble until after the fact that is, when its bursting confirmed its existence."
In other words, investors sometimes find themselves driving down treacherous roads with only their rearview mirrors to...