A bubble is not about how high is the price of something (tulip bulbs, diamonds, dot-com stocks...), it is when the price is not justified. It is a bit for everyone to decide if the price of an asset is justified really, there is not a magic formula that will tell you if it is.
You are right but if you are investing in a company or say stocks then you can inquire and evaluate but how do you evaluate whether the current price of bitcoin is underpriced or overpriced? It's impossible because you cannot imagine who holds how much and what is going on in others' minds. You cannot even evaluate the demand and supply because once again, you just don't know who will sell what amounts, and as far as demand goes, I don't think Bitcoin is in super need or something. I mean there is nothing you can exclusively buy with bitcoins, so demand can't be calculated either.
But I fully agree with you if someone can analyze and evaluate whether the current price is underrated or overrated then it's much easy to make money because you just buy the underrated shares and short sell the overpriced shares. This is how most of the stock market kings operate.