Ie. Prove the rules of divination as written in their belief system.

Traditionally, you prove (or more practically refute) the efficacy of a magical system like divination through empirical consistency.

Divination doesn’t work because it is unreliable. I can predict, say, the next week of weather or the outcome of an athletic game through climatology or sabermetrics far more reliably than the I Ching or Tarot.

But let’s pretend it did work. I’m not sure how you’d functionally prove it.