I don't know of any magic way of testing for that. There are some good signals, of course, such as:
1) pre-interview open source reputation and release. 2) References from people you trust and respect. 3) Publications with significant content written by the candidate.
But I've seen each of these fail, even in combination, in predicting the ability to execute. That said, it's better than nothing.
(disclaimer, I work for Google, and am also waiting for the part about Google in the OP)
One good reason not to work for already big companies like Google and Microsoft, is that your growth potential is limited. Case in point, video.google.com and www.youtubue.com. Nobody needs to wonder about which of the two developers among these projects struck it rich.
Why shovel for someone else's palace when you can build your own?