For starters, just ask a lot of very open-ended non-leading questions.
Let the AI lead the conversation, ask what topics they want to talk about and see how long they can drive the conversation without any leading input from you, etc.

Then start asking about their specific experiences, history, background, opinions, etc.

Try to find places where their narratives don't line up. Ask them about inconsistencies.
Ask them questions that sound reasonable, but actually don't make sense give the previous conversation in context, etc.