Hi,
First and foremost, the thing is building a rapport with the candidate and making him/her feel comfortable to speak out. Because unless he speaks out, we may not be in a position to judge the candidate. Start talking to him, beginning from his family background, i.e., being more generic, and later on his education background, and slowly to his career profile. Ask him to talk more about his company. Make him talk as much as you can and be a good listener. If he is a tech-savvy guy, ask him more about his projects done, show curiosity to know more.
You should also have a candidate assessment form designed where everything gets documented. For example, communication skills, initiative, confidence level, etc.
Thanks,
MTS
This approach would be more applicable for junior and mid-level candidates.