How do you uncover user's mental model?

How many times have you heard someone say something is not usable because it does not fit the user's mental model? The comment may well be right, but how do you know what a user's mental model is?
Indie Young, one of the co-founders of Adaptive Path, has just released a book Mental Models: Aligning Design Strategy with Human Behavior(Rosenfeld Media is the publisher) that explains some methods of uncovering user's mental models.
Chris Baum talked to Indie and writes up that interview in What Is Your Mental Model? - Boxes and Arrows: The design behind the design.
The interview includes links to excerpts from the book and a Flickr set of images from the book. Boxes and Arrow readers also get a discount if they buy the book from Rosenfeld Media.