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.
https://en.wikipedia.org/wiki/Finding_Bigfoot
Wikipedia only has it through nine seasons. And they’ve been regularly called out for “Oh we think we found something!” excited grainy nothingness that gets hyped into an alleged encounter.
The incredulity of the producers is what makes the show work. The Bigfoot hunters are never seriously challenged or meaningfully debunked, and a great deal of the show involves them insisting they’ve parsed mistakes out from “real” Bigfoot evidence.