The task is much the same as interviewing to fill a job — the best way is to do behavioral interviewing, which is to focus on behavior — what the person did the last time s/he…..

One thing to keep in mind: It’s often the case that customers don’t KNOW what they need so asking them is pointless, which is why I suggest that businesses (particularly small ones) create and play the role of TRUSTED ADVISOR

http://customerservicezone.com/customers-need-advice-to-navigate-their-options/