"How do you know that?"
Asking "how" gives answers on the steps and the past that got to this point. It provides facts and events.
It does not provide the beliefs that selected those steps, facts, and events.
"Why" is a beliefs question. Harder to answer and reacher deeper into what people are about. It is not a kind question to ask when there is no trust and safety in the conversation.
This question has worked well for me in the past:
"Why do you know that?"