The TPACK framework has gained popularity in guiding research regarding the assessment and development of teachers' competencies to teach with technology. Since theoretically, the framework is about teachers' knowledge for teaching using technology, effective ways of assessing this TPACK have been a subject of interest among researchers. This paper is about a collaborative lesson design quasi-experiment study whose data collection was done using an observation rubric that was prepared and validated for this study. The study uses six video-recorded sessions to validate the instrument and assesses the level of TPACK of 30 participants. The observation rubric is found suitable to assess mathematics teachers TPACK, but users may need to contextualize it depending on content and technology. Findings of the study also imply that more engaging collaborative professional development activities may be more effective in developing teachers' TPACK. Furthermore, when planning such professional development, it is important to ensure that teachers have access to relevant technology.