We describe our approach to measure teachers' knowledge of students' cognition and present first results of a preliminary survey done with student teachers. This is the initial utilization of our test instrument, which aims to measure competencies concerning the analysis of students' misconceptions and understanding their way of thinking [9]. We invited student teachers¹ to the survey, who have been attending our courses in the past years. We will furthermore introduce the underlying theoretical approach, which we build to design the instrument. This will be substantiated by two example items, which we will describe contentwise and interpret the related results of our initial study. This includes problems and difficulties which arose through the development process.