What I know recently is that calling bitcoin a bubble is not true, bitcoin truly increase price and reduce price, but bitcoin still have one thing in common, if the price decrease today, it will increase back later in a way it will supersede the all time high. Many years ago, people that believe in bitcoin only like to hold, thinking its price will increase no matter what because they have fathom how people will still know the true value of bitcoin. 2018 was the worst year for bitcoin as it decrease below what people can think about, which bring the impression of bitcoin being a bubble to be more solid, but in 2020, after Halving, the price of bitcoin increase and now in 2021 the price increase above $50000. I am thinking calling bitcoin a bubble is not right, the right ideal word is to believe that the price will rise even after it falls.
Even if bitcoin still fall, the way it is, people will still go for it which will make the price to increase. Remember gold is once at this stage before, but people invested in it made it what it is today, the world are moving from old era to digital time, very possible bitcoin will be the most successful asset in the next 30 decades. Believing in bitcoin will help as the price will still increase, even if it decrease, it will return back and increase more than the all time high.
Nobody can tell because bitcoin is not a company. There is no PE ratio or whatsoever. No dividend yields too. If people are willing to pay $60k for a coin then the price is $60k. If people think it is worth $1k then it is $1k.
But there is something else you can watch for:
The stocks market.
If you think stocks are in a bubble right now, bitcoin probably is too. Because bitcoin's movements are kinda correlated with the stocks since the last year or so. As long as stocks keep climbing, bitcoin will also do.