I have a friend who works at the research institute beside the university hospital, and he told me they basically ask a set of questions, while taking a series of measurements via the fMRI machine. They then compare the expected responses with the output from the brain scanner, and can match up the blood flow rates in different parts of the brain with a "positive" or "negative" response. They then perform the inquiries multiple times, until they have a high degree of probability that the questions were answered in a consistent manner.