My guess is as in most cases existing relationships and/or validated relevant past experiences are a great start, but wondering on a ad-hoc basis what the best way to evaluate startup co-founders based on testing their leanness and agility skills; think of it as speed dating format, or a standardized employee candidate test.
Just to be clear, by leanness I mean "lean startup", and by agile, to keep it simple, I mean a SCRUM based product/service development life-cycle. That said, there's nothing new about how important it is to be able to learn and adapt, and it's my understanding that these skills and related experiences have always been valued within the greater start-up community.
Also, while a qualitative methods would be nice, quantitative methods might just be as good, if not better -- in part because this would make it easier to evaluate a wider pool of possible co-founders. I would also guess that this question applies to investors in startups too, since knowing how fast a team is able to learn and adapt is very important in most cases.