You could pair program (Score 1)

Last time I interviewed someone, I did an hour of pair programming with the candidates. I selected something from my regular work that was small enough to take a bite out, and easy enough to explain in little time. So I actually got work done during the interview :D

It was an experiment, but it worked extremely well. At first the candidate is nervous, especially since (s)he is not likely to have experience in pair programming. But soon (s)he settles down and starts to get into it.

The one I ended up hiring was the one that I had to force to stop after one hour. He still wanted to make a little adjustment here, add some functionality there. Easy to tell he was very passionate about his work.

There is no way you can fake the thought process when you have to develop software it in real time. I can recommend this approach to anyone.

So, to anwser the question: you could offer to pair program with one of their developers.