We.. don't have to? The next insane prophet who turns out wrong will rekt the belief.
And evolving Observence might eventually resolve the beliefs over time, anyhow. Metal aversion is actually quite good for ecohippies.
The belief in prophecy is basically impossible to disprove.
If the comet doesn't happen? Well obviously we misintepreted the signs, the oracle is right.
Prophecy from the deranged is far more toxic and difficult to remove than the metal idea.
Look at what he said.
"Black mirror sun spear ash bleeding vein return sky streak."
Black Mirror
-Anything to do with the night sky
-Anything to do with calm waters
-Anything to do with a dark shiny substance like obsidian, soot grease or polished iron
Mirror Sun
-Anything reflective interacting with any bright light.
Sun Spear
-Metal weapons
-'Sun' colored materials like Bronze and Brass.
Spear Ash
-Poisonous ash?
-Some kind of black colored weapon?
Ash Bleeding
-Red ash?
-Poisoned wounds?
-Burnt red rocks?
Bleeding Vein
-Anything that causes bleeding. Massive spread.
Vein Return
-This here would require poetic reading. Return something to the vein of mineral? Payment in blood?
Return Sky
-Oh okay, the gods want their shiny back. Burn it to reach the heavens.
Sky Streak
-Or a comet is coming?
I can come up with the following equally applicable interpretations:
-He is predicting that smelting iron will bring back the comet.
-He is predicting that the spirits want their shiny back and we should stop working with all this copper and iron heresy.
-He is predicting that a volcano is about to erupt.
-He is predicting that iron or bronze will come to kill our people.
-He is predicting that metal can be found at the Black Mirror
-He is predicting that we'd craft a magic artifact weapon.
-He is predicting that for taking the weapons of the gods, we must pay in blood.
-He is predicting that Nomads will attack us soon.
Good or bad, you can 'prove' a prophecy this vague applies. If it missed, clearly the interpreter picked the wrong completely coincidental event.
How do you even disprove it once you believe it has basis?
Simple. You don't believe him immediately.
You set your shamans to scour the heavens. Any comet? No, then it's not referring to comets.
You set your hunters to scour the land. Anything weird? No, then it's not one of the sites affected.
You prepare your stockpiles just in case. Anything goes wrong, you have stockpiles. Nothing goes wrong, then you lost only time.
If you want to prevent a Debilitating belief that only a Golden Age special event would let us burn off, then you accept that it MIGHT be a delusion first,
then you check to see if it's right and prepare anyway.
For that you need to Ignore first, Study second.
Not buy into toxic beliefs for a minor Stability bonus which we were normally going to get anyway.