As introductory physics courses increasingly focus on student engagement, the use of Learning Assistants (LAs) has increased. LAs and other instructors need to have sufficient PCK to effectively guide student learning. Our goal was to research LAs observed PCK in the classroom and to develop a set of questions to assess LAs' PCK. Our research question was: "Is it possible to assess LAs' observed PCK with open-ended written questions?" We video-recorded student-LA interactions in the introductory physics classrooms, developed rubrics to describe the interactions between LAs and students, and developed free-response questions based on the authentic interactions from the videos. Our results are (1) a rubric to assess LAs PCK in video recordings or free response written assessments and (2) a set of open-ended written questions that assess LAs PCK. The LAs were dominant in one or two categories in both the written and video coding, making it possible to draw conclusions about their PCK. We answered our research question by demonstrating the development and validation of a set of questions to assess LA's PCK. Our rubric can be used to analyze both videos and written questions by supervisors who are interested in assessing their LAs' PCK.