We describe a methodology for identifying evidence for the use of three types of scientific reasoning. In two case studies of high school physics classes, we used this methodology to identify multiple instances of students using analogies, extreme cases, and Gedanken experiments. Previous case studies of expert scientists have indicated that these processes can be central during scientific model construction; here we code for their spontaneous use by students. We document evidence for numerous instances of these forms of reasoning in these classes. Most of these instances were associated with motion- and force-indicating depictive gestures, which we take as one kind of evidence for the use of animated mental imagery. Altogether, this methodology shows promise for use in highlighting the role of nonformal reasoning in student learning and for investigating the possible association of animated mental imagery with scientific reasoning processes. (Contains 7 figures, 2 tables and 5 footnotes.)