When conducting job interviews, you will have to ask tough qualifying questions if you want to find an outstanding candidate. There is a human tendency not to ask the tough questions for fear they will offend a candidate. However, if you introduce the question properly, both you and the candidate will benefit. For example, perhaps you say this: "Do you mind if I ask you some questions that may be tough to answer? My intent is to make sure you would be comfortable joining our organization." Outstanding candidates will give you favorable responses, since they're not interested in talking about mundane subjects.
Excerpted from Attracting & Rewarding Outstanding Employees