Perhaps by checking what a Ponzi scheme actually is and seeing whether bitcoin matches the criteria?
In my humble opinion its current price is
partly based on a pyramid schmeme...but that is the property of every bubble
So now it it not a Ponzi but a pyramid schmeme?
No, a pyramid scheme is not part of every bubble.
The worst you could accuse bitcoin of is being a pump-and-dump (but you could make that accusation of any investment vehicle).
EDIT: bottom in?