With this question, you'll explain that you're only interested in helping this person be the best sort of employee they can be (which is the truth). Your reference here will respond not so much with weaknesses of the candidate, but rather with a description of an environment in which they'll be successful and the sort of support they need. From this, you'll have a clearer idea about whether the individual will fit in to your culture and if your workplace is one that will enable their productivity.
Most people here will respond with "Oh, I'd say an 8 or a 9", at which point you fire back with "Why not a 10?". Now you've illuminated in the reference's mind the gap between the person as the co-worker they knew and the person as a perfect employee. The explanation for this gap will be an begrudging admission of where the candidate can improve.
Draw on the reference's experience of the candidate all the way through the hiring cycle to get a sense of how they are in interview mode vs. every day work mode. Answers here will range from something that pleasantly surprised their previous co-worker, or at the very least something you can then push the candidate on in future interviews.
Remember, no single reference is enough to paint a full picture of your candidate. Getting any sort of reference, whether overtly positive or negative, simply means you have more calls to make, and these follow up calls can also be about previous references. If you speak to two different people who worked with your candidate at the same company, ask each if the other was truly privy to the candidate's work style to learn how much you can weigh their review.
What are some of your techniques in reference calls? Leave a comment or tweet @EnteloRob!