I'm always looking for a co-founder, whether I'm the geek or the suit. When a suit approaches me to be the geek for their idea, I usually give them some simple technical task to do - like setup a Tumblr or a Twitter account for the idea. Then I'll often ask for something businessy - form a C-corp for the idea or file a provisional patent.
Finally, before I even consider opening my laptop to code their idea, I want some paltry measure of idea validation. Are you selling a product? Good, find someone who will pay you to do the task manually before we program it. Are you looking to give the service away and monetize the user base? Good, get 5000 emails from a landing page describing your idea, or get 1000 followers on Twitter for your idea's account.
If they can do all this in a day or even a week, then they're quality co-founder material. Any longer and it's a judgement call. Most won't get past step one, and you certainly didn't want them as a co-founder.