Actually, a lot of the time, I don't.
If the person I'm interviewing is really solid, I already have access to code samples and other work when they show up.
But that's the exception.
With the really talented people, I can tell by the way they talk about their past projects. Its usually pretty easy to know if someone has really gotten into the shit before or if they're sandbagging it.
But sometimes, someone shows up and doesn't bring code samples, or balks when I ask for some, or I can't get a good read on their past work.
So they get a code challenge.
Its always something stupid (yeah, like Fizz Buzz). Something that shouldnt take any longer than the time it takes to write it down.
I fully recognize that this is not a good indicator for whether or not someone will write good code if we hire them.
But it sure is good for weeding out people who shouldnt have made it to the first interview to begin with.