Assuming that you have already checked them out for culture/personality fit and development skills, I would suggest doing the following:
Ask the candidate to describe the architecture of a medium-to-large scale system that they actually designed and/or implemented. Ask probing questions around the design tradeoffs they made.
Provide the candidate with the design of a system that has more than a few known flaws. It can be one you made up or one that is well-known to all. Ask the candidate to identify the flaws and redesign the system, assuming they had no time or money constraints.
Present the candidate with a set of requirements for a non-trivial system (i.e. a distributed system for reviewing medical images in a hospital environment) Have them whiteboard the components of the system in increasing levels of detail until you feel comfortable with their ability to elicit requirements from the stakeholder (you) and make appropriate tradeoffs in their design.
None of this will guarantee that you'll select the best "architect" (I have the same issue with the title architect as Joel and Jeff do). However, these are definitely good filtering techniques and should help you identify someone with worthwhile characteristics for your team.
