When teachers have a practical, easy to use tool to self-assess acts of teacher leadership, they are better equipped to develop as leaders by self-determining to whom or what they are committed (relatedness), what they know and can do (competence), and when and how to act (autonomy). The purpose of this mixed methods study was to assess the validity of the Five Features of Teacher Leadership Framework and Self-Determination Guide (FFTL) through field testing by credentialed teacher leaders. Twenty-five credentialed teacher leaders representing all six regions of the United States participated in the study. The FFTL was favorably viewed by all 25 teacher leaders, revealing reasonably strong confidence in the tool's face, content, and construct validity. The findings suggest that participants considered the FFTL credible enough to trust as a guide for self-determining acts of teacher leadership. By describing an act of teacher leadership and using the FFTL to self-determine the degree to which the act accomplished the five core features of teacher leadership, the teacher leaders in this study were empowered to grow and develop as leaders privately, in their own way, and at their own pace. More research is needed.