Since the interview process doesn't include any sort of personal question or assessments other than one's ability to memorize algorithms and such, How do you measure "culture fit"?
(I am an ex-Googler)
It's very subjective and falls clearly in the category of "You recognize it when you see it".
Typical example of a hiring committee deliberation at Google: "He didn't find the optimal solution but he looked very excited about the problem when he was trying to solve it".
Enthusiasm, interest, being personable, excitement about solving problems, all these things count when you interview, especially at Google.