Does his/her working style match your expectations-- you need first to figure out if, at the extremes, you want to be intimately involved in every decision, or if you want the architect to design and deliver a turnkey work of art to you, then you need to ask references how the architect worked with them and figure out if there is a match.

Open ended questions are good-- tell me about the strengths/weaknesses you found, did you have problems and how were they resolved, would you work with him/her again, what did you like/dislike about the interactions.

Specific questions about timeliness, responsiveness, listening skills, imagination, flexibility, fairness, etc are also good.

There's not a right/wrong, good/bad answer, its a matter of matching needs and expectations. One person's perfect architect is another person's nightmare, so it's really important to analyze and be clear about your needs and desires in the interaction before you start.