I had been interviewing people for last 3 to 4 years. There are various qualities that you look for but sometimes for an experienced position you look for some good knowledge. This is a simple trick that has worked for me. This will work as this is scientific :). Say if you are interviewing a person for SEO expert position:
You: Give me the top 3 tough areas from Search Engine Marketing/Search Engine optimization that you feel you have mastered it.
Ok, it tough but let me get some,
- Trust rank.
- Link Building.
Now ask him to explain trust rank to a layman. If he is able to explain it in simpler terms then you can be sure of his understanding but if he tries to complex it with Google complex algorithm, then call the next candidate. Generally a good candidate will start from the basics of rankings (in very short sentences) and will try to fit in the role of trust rank in it. Why it was needed? How it is helping Google to achieve the goal etc will be a part of his explanation.