From what I remember, in past bubbles that burst, everyone thought stocks will go up forever. So, if people are wondering out loud (with widespread traction) if there's a bubble, there's probably not one. It may be self-fulfilling. The awareness of a potential bubble influences us to be more cautious.
For the last most explosive bubbles -- "dot-com" (~2000) and "housing" (~2008) -- you can go back and find all sorts of warnings and critiques and fretting and skepticism in newspapers, academic and industry journals, blogs, television, personal ephemera, etc. And not too far from almost all the examples you find, you'll also find somebody blowing off the skepticism.
The tricky part is that you can more or less find similar dialog in all those sources even when there didn't turn out to be a looming collapse. There's just always people worrying about collapse or hard correction when the markets are running hot and always people blowing them off, and sometimes worriers prove right. Eventually. And sometimes they don't, and the underlying economy either catches up with the market or government policy successfully props it up for long enough that some other concern dominates or some other industry can absorb the over-allocated investment.
Basically, it turns out your heuristic for spotting a bubble faces too much noise and isn't actually all that informative at all.