Charles Dickens died before finishing The Mystery of Edwin Drood. He didn't leave any notes, so many literary critics have tried to guess the ending.
One critic did something remarkable. He predicted, just from the book, that a certain law had been passed in Turkey in a certain specific year. A search through 150-year-old Turkish legal records showed the prediction to be correct.
Is that proof the critic knows the ending? No, but then, science done in laboratories isn't "proved" either. The point is that the critic exposed himself to the chance of being wrong. People have more confidence in theories that pass such tests. After several successful predictions, we could wind up having tremendous confidence.