If you predict 100s of events, then use a method like Brier scores, you can get a good idea of how good you are. Even then it's probabilistic, but with enough samples it becomes incredibly unlikely you are just the worlds luckiest guesser.
> it becomes incredibly unlikely you are just the worlds luckiest guesser
In any case, we are not talking about _you_ becoming the world's best guesser, we are talking about _somebody_ becoming the world's best guesser. That is going to be someone, so no need to be surprised when they emerge.
so I think this comes down to how the predictions were arrived. One way to do this is to ask individuals who both have similarly high predictive scores and bet on the same types of events to explain some of their past predictions, and if their methods are similar then you've learned a new predictive tool.