This paper contains a collaborative video-based approach to foster prospective teachers' diagnostic skills with respect to pupils' L2 reading processes. Together with a peer, the prospective teachers watched, systematically selected, analysed and commented on clips from a comprehensive video corpus on L2 reading strategies. In order to assist the prospective teachers in this demanding process, a "Standardised Clip Protocol" (SCP) was developed and tutorial support was provided. The SCPs were subjected to various empirical procedures. Quantitatively, the frequency of all learning challenges mentioned by the prospective teachers was counted. Qualitatively, the nature of the diverse reading challenges identified by the prospective teachers was investigated. In that respect, a focus was put on the prospective teachers' diagnostic findings regarding pupils' lexical and conceptual awareness. The results indicate that the skills to notice and diagnose L2 learners' lexical and conceptual reading challenges form an essential part of "Teacher Language Awareness" (TLA) and should be integrated into teacher education programmes.