"To trade with an asset at a price or range of prices that far exceeds the intrinsic value of the asset."
So, to make sure if cryptos are in a bubble you have to determine what their intrinsic value is.
(Or, if classic intrinsic value can be substituted with other value, fundamental value, like network value, or usage value, utility.) Seems to be almost impossible, although there were some guys trying to calculate fundamental values of Bitcoin.
Hello appreciated @deathcross.
In my opinion, the main value would be practical use. That flexibility of the crypto which the Fiat can never have. P2P transactions without commission, or with very low commissions are invaluable. Transfers that the established borders disappear, the free convertibility. All these are the values that can help to establish the range of commercialization.