I define a bubble as unsustainable conditions. Examples such as prices rising higher than they can stay long term, or demand that is significantly above normal.
The problem is that knowledge of a bubble is best seen in hindsight.
People say that houses in some markets command high prices because of demand. Sure, but is that demand real, or is it because of FOMO?
I have read that one way to verify is to use the rent in the same market. If you cannot buy a home with a mortgage, and rent it out to at least break even, then the demand is not real. If the rent is low compared to home prices, then it means that people there do not make enough to buy the homes nor to pay the rent, and high prices are not sustainable.
I recall the superhot home market in the metropolitan Phoenix in 2005-2007. A friend of my mother visited her from DC. The friend was buying a new home in Anthem (north of Phoenix, on the way to Sedona) to flip. She eventually ended up stopping payment, losing her down payment, and letting the bank foreclose it.
Back then, people justified the market by saying so many thousand peoples were moving to Phoenix each month, and they had to live somewhere, blah blah blah... There were stories of realtors and developers bringing in out-of-state speculators on buses, to let them view the new subdivisions and to buy, buy, buy...
I recall my mother told me that I and my siblings were not knowledgeable enough to take advantage of the situation despite being natives to the area. Instead we let "outsiders" come in to get all the good deals.