When it comes to decide if there is a bubble, you have to think about how much is worth to you. How much would you pay to get, e.g. a 4% in dividend instead of spending the money today in a car or a TV? How much would you pay for a diamond or even a soccer ticket? (my personal for those two is quite low)? Or, How much would you pay for 1 in 21 million bitcoins?
Getting to the point, bubbles happen when you start buying something just because you think someone will pay more for it, instead of looking at how useful is it for you or for a community of people. Buying something is actually telling the market that they are wrong, and what you are buying is worth more than what the consensus means. This is being contrarian, anything else is speculating and creating bubbles.