The fundamental question of rationality is not, despite widely held opinion, "why do they believe what they believe?" but rather "why do I believe what I believe?" This is closely related to the notion of a stack trace, and can be abbreviated by asking the question "whence?"
Other questions about beliefs:
- Do I endorse the process that formed this belief?
- What are the consequences of holding this belief?
- Does holding this belief make me exploitable?
- If I didn't already have this belief, would I press a button to install it?
- What am I precluded from doing as a result of this belief?
- What is this belief made of? (images, words, felt senses, relations)
- How much would I be willing to bet on this belief?
- Is this belief falsifiable?
- If the belief were wrong what, concretely, would I expect to see in the world?
- What values am I trying to express with this belief?
Surprising beliefs elicited by belief reporting are especially good targets for these questions.