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What we'd ask depends on the context. In general, not all rationalist teachings are in the form of a question, but many could probably be phrased that way.
"Do I desire to believe X if X is the case and not-X if X is not the case?" (For whatever X in question.) This is the fundamental lesson of epistemic rationality. If you don't want to lie to yourself, the rest will help you get better at that. But if you do, you'll lie to yourself anyway and all your acquired cleverness will be used to defeat itself.
"Am I winning?" This is the fundamental lesson of instrumental rationality. It's not enough to act with Propriety or "virtue" or obey the Great Teacher. Sometimes the rules you learned aren't applicable. If you're not winning and it's not due to pure chance, you did it wrong, propriety be damned. You failed to grasp the Art. Reflect, and actually cut the enemy.
Those two are the big ones. But there are more.
Key lessons from Bayes:
Others I thought of:
I'm not claiming this list is exhaustive.