The point of a job interview is to make sure the candidate has the necessary skills to do the work. But aside from references, how can you determine if a potential employee actually knows what he claims to know? Here are two tips for finding out how deep a person's knowledge and experience goes:
Ask "how" and "why." When an interviewee relays a success story, drill down to understand as much as you can. Keep asking, "How did you do that?" or "Why?" until you get to a question the candidate can't answer. This isn't always comfortable but it will shed light on his true capabilities.
Bring in a fellow interviewer. With two people conducting the interview, one of you can focus on asking the right questions and the other can listen attentively to responses.
By Michael Mauboussin. Harvard Business Review