I'm not in the industry. But I know a thing or two about artists. And that most of them don't have discipline. Creating was their hobby, and when it becomes their job, they completely lose motivation. You have to look for people who are regimented. People who have structure to their lives. Once you stop being wowed by the art, then you can start to get a sense of an artist's personality. Be careful of people who are too laid back. They're unable to handle stressful situations, and their laid back attitude is just a coping mechanism. They'll crumble under pressure.
Also, don't value passion for the project too highly. Because passionate people can let you down too. Again, it's just the lack of discipline and the stress causing them to crumble under pressure. It doesn't matter how much they like your project, if they can't cope with showing up to work consistently, then they won't get the work done.
And just like the other anon said, you want to find guys who have a few notches under their belt already. Guys who have completed small projects. So you know they're capable of following through. Of course this way of selecting still isn't foolproof, but it's a whole lot more reliable than looking at portfolios. Everyone puts on the best representation of themselves in a portfolio. Hell, have you ever seen advice for putting together a portfolio? It's all deception.