A bubble is when everyone is talking about cryptos, e.g. your taxi driver, the bellboy, etc. Outside of my close circle of tech colleagues zero people are talking about Bitcoin.
A bubble is when something's cost differs from its intrinsic value - a bubble can happen on something which is shockingly insignificant.
Bitcoin's "intrinsic value" is extremely hard to define, so we could just call a bubble whenever the price rises extremely dramatically without much reason (such as now)
I don't think that it is really hard to do
It may be hard to estimate or assess, but this is a different task, anyway. Bitcoin intrinsic value comes from its transactional utility, i.e. its value as a currency and payment system, and this value Bitcoin would have if we removed it from speculation entirely. How much Bitcoin would cost in that case is unknown, but I don't think very much. Just in case, the value that Bitcoin has for being used a speculative vehicle is certainly not its intrinsic value. In this way, it could be safely claimed that Bitcoin is highly bubbled nowadays