After years of recruiting creative people for advertising agencies, I still don't have a foolproof way of telling who's going to be great.
Looking at my own "system", it tends to be a 60/40 proposition. Forty percent has to do with what's in their sample books. The 60 includes a lot of foo-foo dust.
For one thing, a lot has to do with how the person will do in the agency culture. Are they fun and enthusiastic? Are they motivated to work on their own, or will they need a lot of hand-holding? Are they open and enjoyable to talk with?
Importantly, do they easily share their feelings? Ad agency creative departments are little hot-houses of emotion. Some of it goes into the ads and commercials. The rest spills over. Or gets pent up.
Another measure I've always used is strictly unscientific. Do I like them? I've rarely hired someone I didn't like, or at least wasn't prepared to. In advertising you practically have to live with your colleagues, days and sometimes nights. Nobody can avoid anybody.
One quality that I've always looked into is the ability to process information quickly. I've found it a useful way to get a handle on a person's intelligence.
I have to add that everyone is nervous on a job interview, and I try to discount that. That's where their portfolio of work comes in. Is it nervous, too --- or calm, confident, funny, and authoritative? Is it surprising and fun to look at, so you want to keep going, page after page? Do you secretly wish one or two of those ideas were yours? That's the ultimate test.
Think of it this way. Creative directors don't need writers or art directors. They need two things: ideas and help.
Be prepared to do more than you're paid for.