There is extensive evidence that active learning works better than a completely passive lecture. Despite this evidence, adoption of these evidence-based teaching practices remains low. In this paper, we offer one tool to help faculty members implement active learning. This tool identifies 21 readily implemented elements that have been shown to increase student outcomes related to achievement, logic development, or other relevant learning goals with college-age students. Thus, this tool both clarifies the research-supported elements of best practices for instructor implementation of active learning in the classroom setting and measures instructors' alignment with these practices. We describe how we reviewed the discipline-based education research literature to identify best practices in active learning for adult learners in the classroom and used these results to develop an observation tool (Practical Observation Rubric To Assess Active Learning, or PORTAAL) that documents the extent to which instructors incorporate these practices into their classrooms. We then use PORTAAL to explore the classroom practices of 25 introductory biology instructors who employ some form of active learning. Overall, PORTAAL documents how well aligned classrooms are with research-supported best practices for active learning and provides specific feedback and guidance to instructors to allow them to identify what they do well and what could be improved.