This paper proposes an analytical tool for the evaluation of prospective mathematics teachers’ (PTs) diagnostic competencies through noticing critical incidents in hypothetical classroom situations (mathtasks). Data were collected from nineteen PTs attending an undergraduate course for one semester. Data analysis highlighted variations within the four characteristics of teachers’ diagnostic competencies that are described by four internal levels inspired by how teachers notice. This analysis resulted to the analytical rubric we present and exemplify in this paper. We see the potential of the proposed rubric in research and mathematics teacher professional development.