In practice, I try and hire people that have blogs wherein they showed knowledge as well as serious and deep thinking about the type of problems I want to hire them to solve.
The only person I've ever truly thought did an excellent job I hired this way. I didn't know him. I didn't look at a resume. I had one conversation with him. I just read his blog, and I could tell from his writing that he would do a good job. Actually, his blog was just his fiction writing and he was a UI designer and coder but I could still tell from his thinking. He was so smart--a structured, logical, deep and creative thinker. It was clear to me that he would do a good job. He did. I tend to be a perfectionist, and he was the only person I've ever hired that did a perfect job. I had absolutely no criticisms about him. He always exceeded my expectations. It was amazing. I love working with the best people--nothing is more fun.
If they didn't have a blog, I'd tell them to write down their thoughts every day for a few weeks and then I'd read it. I believe that you can't tell anything about anyone except how they behave over time. People lie and misrepresent themselves. But it's pretty much impossible to misrepresent yourself over a period of time.
I might try and give them a few of my problems to work on as well. But, if someone isn't the quickest thinker and can't answer a problem I give them in an hour-long interview I really don't care. What if they're amazing problem solvers but they just take overnight? One of the smartest people I know is like this.
With a blog, or writing over time, you also get a sense of how well they can articulate their ideas--if they have amazing ideas but they can't articulate them well, then the quality of their ideas doesn't matter. If for some reason they're terrible writers but still smart, you can just have them create a podcast. If they can't do either, then they can't articulate themselves enough to be useful to a company.
Watching their thinking over time, you can see how many ideas they generate, since you don't just want people that will only solve the problems you have. You want people that see things you can't and generate ideas themselves.
I don't care about how well people fit in. A start-up isn't a clique of best friends, although you certainly need people that you can be bluntly honest with and can be bluntly honest with you. It's better if you like them, of course, but I'm never trying to build any kind of culture except a culture of excellence--well, excellence on deadline.
Another thing I've always wanted to try is just to hire someone for two weeks and give them non-critical tasks to do. I could see if they met deadlines and how they actually worked. The best way of finding out if someone can do something is to watch her do it.