The diagnostic competence of foreign language teachers includes the ability to judge oral language productions. However, these are difficult to judge because of their complexity. Video-based data can be used to explore the diagnostic processes when judging oral productions. In the present study, prospective teachers evaluate nine video samples twice – directly after first viewing the video and after using a rating scale regarding the linguistic features. This study examines the extent to which prospective primary and secondary level teachers use different information to make a judgement compared with experts. The results show that linguistic features are incorporated to varying degrees into the formation of judgements and vary according to pre-service teacher training. Besides the focus on linguistic features, a high proportion of unexplained variance remains in the judgements of the prospective teachers.