Students' sense of belonging predicts their success and persistence in STEM courses.  Collaborative, small-group activities form the foundation of many research-based instructional strategies. Our broader project seeks to understand the role of small groups in students' sense of belonging to support instructors in the formation of equitable groups in active engagement classrooms.  In this article, we focus on the construct and discrimination validity of a belonging measure. To assess the belonging measure's ability to discriminate across time, courses, and demographic groups, we administered a short survey on belonging in a variety of STEM courses that used groups as a pre- and post-class assessment.  We analyzed the results using structural equation modeling to inform the validity of the survey and identify possible differences of interest.  The results provided evidence for both construct and discrimination validity. Belonging varied across the courses and changed from pre to post in two of the four courses: one course saw a decrease  and the other course saw an increase.  Men tended to have a higher sense of belonging than women and the changes in belonging increased these gender differences.  One possibility is that the differences observed across courses could result from the different practices used to support group work within each course. The validity evidence for the belonging measure indicates it will support our ongoing research to establish the statistical relationships between instructor practices to implement and support small groups and students' sense of belonging.