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
Came here to basically say just this, completely agree with you.
I think that in theory, it would even be possible to get pretty exact data on how much intrinsic value Bitcoin actually has, by looking at all transactions and sifting out the ones that are tied to anything other than buying, selling or holding for pure speculative purposes.
In practice, it's pretty hard to do since it would be fairly hard to actually tie transactions to specific purposes, despite everything being recorded on the blockchain.
About Bitcoin being in a bubble these past weeks, I think that's pretty obvious, it's not like actual use of Bitcoin has suddenly skyrocketed, apart from Japan perhaps.
Certainly not in a way that warrants this sudden rise in price.
that is an outstanding thought,looking at the entire market i could see many alt coins popping their bubble and keeps on coming back and even bitcoin is trying hard to cross $3000 but the bubble keeps on bursting. 
