The "AssessToday" observation protocol was created to measure teachers' use of short-cycle formative assessment through observation of teachers in a single instructional period. This classroom observation instrument utilized seven dimensions: "learning target, question quality, nature of questioning, self-evaluation, observation" of "student affect, instructional adjustment", and "evidence of learning". Each dimension is rated using one of four proficiency levels that are based on mastery. The purpose of the study was to examine the construct validity of the classroom observation protocol for middle and secondary mathematics teachers. Evidence is provided for content validity, criterion-based and interrater reliability, and exploratory factor analysis. Implications for using the instrument for research and practice in professional development and improvement of individual teacher use of short-cycle formative assessment are discussed.