How do one know whether an appreciation in the price of an asset is a bubble or not? For example, during the 2007-2008 crisis, there was a housing bubble, which is only mainly agreed upon when the bubble actually starts to burst. Beforehand, however, the rise in housing price was not commonly accepted as a bubble, especially during the period when the housing price was still rising before the crisis.
So how do we know whether an appreciation is a bubble or not?
Any suggestions of articles about the issue are appreciated.