Passive or active?

"Coolfacts", an application for the iPhone, suggests, as Fact #71, that the vocabulary of the average person consists of 5,000 to 6,000 words. How do they know? And how would you go about working out what your own vocabulary is - especially as we have two types of vocabulary - active (the words we use all the time) and passive (the words we know but don't use on a daily basis). Do you record yourself and count the words back? Or is there another way?