Complex competences, such as managing students' aggressive behaviour, are challenging to develop during teacher training. Recently, video-based simulations have been considered promising, yet suitable assessment instruments are limitedly available. This paper reports on the design and evaluation of a video-based assessment tool tailored to measure preservice teachers' verbal aggression management competence (V-AMC) development. Design considerations for video-based activities served as a framework, resulting in an assessment instrument consisting of video vignettes mirroring classroom situations concerning verbal aggressive behaviour, questions prompting situation-specific skills development, and a coding scheme to analyse preservice teachers' answers. Furthermore, this paper discusses the results of the instrument's implementation on preservice teachers' (n = 182) V-AMC. Findings support the appropriateness of the instrument in assessing V-AMC levels and mapping the impact of a simulation-based intervention. The data partially confirm the intervention's contribution to V-AMC development. Implications for future interventions targeting competence development are discussed.