How can educational developers best formatively assess impacts of their services? Standard practices tend to rely on indirect measures, such as counts of participants and feedback surveys. This paper responds to recent calls for more robust approaches directly measuring outcomes. We describe how to implement a new, transferable, evidence-based approach for directly measuring instructors’ learning gains within and across educational development workshops: the Thin-Slice Embedded Direct Assessment (T-SEDA) Process. Although this approach does not measure longterm retention of learning or effects on future teaching behaviors, it significantly enhanced our toolkit for formatively assessing educational development workshops. Through case studies, we illustrate principles underlying our approach, impacts on instructors’ learning, and how we iteratively refine our programs and practices using the T-SEDA process. We also discuss lessons learned for fostering an inclusive, collaborative culture of formative assessment among educational developers.