We discuss research involving the Conceptual Survey of Electricity and Magnetism (CSEM) to evaluate one aspect of the pedagogical content knowledge of teaching assistants (TAs): the knowledge of introductory students' alternate conceptions in electricity and magnetism as revealed by the CSEM. For each item on the CSEM, the TAs were asked to (1) identify the most common incorrect answer choice of introductory physics students and (2) predict the percentage of introductory students who would answer the question correctly in a post-test. Then, we used CSEM post-test data from approximately 400 introductory physics students (provided in the original paper describing the CSEM) to assess the extent to which the TAs were able to identify the alternate conceptions of introductory students related to electricity and magnetism. In addition, we conducted think-aloud interviews with TAs who had at least two semesters of teaching experience in recitations to explore their reasoning about this task. We find that the TAs struggled to think about the difficulty of the questions from introductory students' perspective and they often underestimated the difficulty of the questions. Moreover, the TAs often expected certain incorrect answer choices to be common among introductory students when in fact those answer choices were not common.