You really can't. There are some people who have a skill for talking about what they've seen others do, and for memorizing what others said. If that person has sat in a post-mortem for some failure, they'll be able to tell you all the things that went wrong, how "they" fixed them, and why. And yet they can't code.
I'm not hiring people who can talk and not code. I'm hiring people who can code (and, ideally, talk). The only way to verify that they can code is to witness it happening.
Well, in that case, what you definitely want to do is optimize your entire interview process around the 1% of people who can pull this off, instead of just firing them when they can't execute on the job.
I'm not hiring people who can talk and not code. I'm hiring people who can code (and, ideally, talk). The only way to verify that they can code is to witness it happening.