Teachers must often analyze and select the educational resources they consider relevant for their students, being textbooks one of the curricular materials of preferential use. A textbook lesson shows the instructional process planned by the author to promote the learning of a given content by potential students, so it is essential that the teacher is competent to analyze and assess what happens in this process. Reflection on the relevance of a lesson provides didactic-mathematical knowledge to guide the teacher in making decisions about the management of the text. This paper describes the design, implementation, and results of a training action with 45 preservice teachers, oriented to the development of reflective competence through the analysis of the didactic suitability of a lesson on direct proportionality. In the initial evaluations of the lesson made by the preservice teachers, we found some features of didactic suitability indicators in different components. However, the reflections elaborated by the participants are vague or ambiguous. The results of the implementation show an evolution in reflective competence on the part of most of the preservice teachers, who were able to make a detailed assessment by correctly applying suitability criteria, mainly in the cognitive-affective and instructional dimensions. The participants recognise the importance of the training received to guide their reflection on teaching practice, which they need to complement with content-specific didactic-mathematical knowledge to achieve adequate competence in analysing instructional resources.