You cannot. The operator could easily invest their own money into the ponzi from various addresses, knowing that they would have the money returned to them even if they have to payout to other investors. If they do not receive enough money then they simply run away with their own money plus the money of their other investors.
Dont you think that the same probability exists for a dice site as well that accepts invetment in bank roll ? If players win against their odds, they can also run away with all the investments. They can play in their own website knowing the seed. In fact, Dicebitco.in did the same. In Ponzi game, there is no seed. The probaility is 100% real - "Whether there will be a next investor or not". So, how can it not be provably fair ?
You need to have a very highly trusted person to be running a casino to invest in their bankroll. The vast majority of the time it will not be a good idea.
You are exactly right that there is no seed in a ponzi game so it would be that much easier to cheat.
The point is that after a ponzi starts to payout enough then people then it will attract an ever increasing amount of money which is not the case with casinos. The ever increasing amount of money will cause the operators to have a greater chance of running away