What are the most common methods of determining valuation used by economists that claim Bit-Coin is on the verge of being a bubble
There are two ways to look at the value of a financial instrument. One is the underlying asset base, the other is the yield.

Bitcoin has no underlying asset base and no yield.

Therefore, its only value is whatever people imagine it to be.

In other words, it's completely overvalued. But that does not mean it is on the verge of massive corrections. There is an old saying (attributed, as they often are, to Keynes) that the market can remain irrational for longer than you or I can remain solvent.