It's far from a universal technique, but you can tell a lot about someone's sincerity and true skill level by seeing books or blog posts they've written, their open source contribution and projects, their Twitter account, screencasts they've made, citations to talks they've given, and what not.
Geeks cut from all cloths who want more doors to open in future need to keep their digital "footprint" in mind and strive to maintain and build a good image and do great public work.
His technique for hiring programmers: (unpaid) probation for 30 days, no exceptions. He almost always confirms sooner than that, but 30 days is the limit for someone who isn't working out.
edit: to stress: I AM NOT recommending this as general business practice.
I should have mentioned that most programmers who look to join are grad student aspirants who want to enter the institute's prestigious PhD program via that route. The alternative to that is the mainstream route which has an entrance exam and a huge amount of competition.
The other thing is that this is a world-class research lab in an area attractive enough to hacker-types that the young people who apply are not too worried about the money. These are of course young folks with no dependents. You get free or cheap housing and food, and access to a wonderful library, etc.
It's about as exploitative as grad school, let's just say. This of course voids any suggestion you might think I made to adopt this as a general business practice.
This puts a serious constraint on the jobs an applicant can try at - even if one is OK with trying out for a job for 30 days without pay, far fewer people can afford to do a few of these in a row and choose the right job for themselves.
IMHO, it is perfectly reasonable to insist that one gets a "try-before-buy" option. Tacking on the "unpaid" factor, sounds like a "While we are at, let us screw them over too" afterthought.
Personally I think most arrangements in academia/higher education are indeed exploitative.
If you try it with people, make certain the employees understand the background, otherwise they might be out the door later -- and you'll have to eat the cost of hiring again.
This might work on me even if I wasn't desperate -- and you e.g. showed me the books:
"This is a startup, we are optimizing for runway right now. If we like you, we will pay you and give you options -- but we must be thrifty for X months."