>What are your bubble indicators?
Thanks for motivating the question.
For me (joe blow here, not a pro), watching in my lifetime, the question to ascertain this would be:
Is there large scale lack of money coming to the table to address the market size in the asset class that has bubbled (grown upon itself in size, self-feeding) ?
First of all, you have to be comfortable with this above arguably limited definition of mine for a bubble. The prices in the asset class can go down for different reasons other than this above.
Living through 2000 tech bubble, I recall the peak took place over about 6-8 weeks. An easy indicator of lack of money coming to the table on the large scale to address the market size that had bubbled was IPOs that were scheduled were paused or canceled during this peak period.
The 2008 real estate mortgage crises was a little tougher to see for me, not working or paying attention to the banking or real estate sector, but reading a bit, looking in hindsight, banks were getting hesitant to lend to one another as defaults were rising -- the key here being lack of big $ willing to support the assets. Defaults on the residential market were already two-fifths through the upswing when Lehman failed at the end of Q3 2008 and things snowballed from there, propagating to other asset class declines.
https://fred.stlouisfed.org/series/DRSFRMACBSSo, here again it was moreso the lack of institutions applying new money to the asset class that tipped things. But again, it is seeable if watching, because the tipping point wasn't happening at the mom and pop level, rather whether big bank-to-bank lending was continuing or stopping.
In view of this, what to look out for now would be what is happening in commercial real estate? It could be a problem, virus causing lack of commercial occupancy, but the sector issue probably does not constitute a bubble (grown upon itself in size, self-feeding).
These are the two main crashes in my lifetime I can comprehend first hand. The crash of '87 (easily recovered technical correction--it is my lifetime, but I'm not counting that).
Does this apply to events outside of my lifetime; tulips (maybe oversupply to demand on something without intrinsic value) or 1929 (maybe, leveraged run up--looking for calls on margin might be too late an indicator) , I don't know.