In the beginning when I had to interview, I was always uncertain. How to determine the quality of the candidate, how to review, how to bring out the best in people - everything to me was just gut feel and guess work. But my boss insisted that I do it anyway, and she will provide the backup support being the 2nd interviewer.
So I went ahead, and most of the time I was unable to make a judgment about how good people are even after I've interviewed them. I would do a quick summary of the person and let my boss see him too. Then we will reconvene and she will tell me what she has picked up from her observation. I learnt to read cues, verbal or non-verbal, and asking critical questions that will help open up the candidates and help us understand them better.
Over the course of a few interviews, I found that I knew better already. I can now make a quick judgment, and profess my opinion confidently when I am recommending for 2nd interview, or cut short the interview and let the candidate go. I found myself confidently expressing: "I like this candidate." Or "No, Boss, you don't need to see." It's amazing! Although I didn't like interviewing at first because I was always so unsure, now I found that the extra interviews that I do actually hone my skills.
Of course, interviews are not foolproof. There will still be hiring mistakes. But my boss has been through enough interviews to make the odds better. And it is great to be learning under her guidance. I still learn a lot from her, especially the 50-50 cases.