Diagnosing is one of teachers' key competences. Lately, a new model of teachers' diagnostic competence and therewith a new measurement method focusing on diagnosing students' learning behavior in an interdisciplinary approach have been developed. They can build a basis for promoting teachers' diagnostic competence. However, some questions relevant to promoting teachers' diagnostic competence concerning students' learning behavior still remain unanswered: Which values do teachers-on-the-job in contrast to preservice teachers with or without teaching experience gain through experience and which are the relevant variables that lead to a high level of diagnostic competence focused on learning behavior? Thus, this study aims at enlightening these issues. In a sample of 293 participants (93 German grammar school teachers, 107 German teacher students in their second phase of teacher education, and 93 German preservice students in their first phase of teacher education), teachers' diagnostic competence concerning learning behavior as well as teachers' motivation, attitude, self-efficacy, knowledge, and reflection on experience concerning diagnosing was measured. Motivation, attitude, and knowledge were found to be substantial positive predictors of diagnostic competence concerning learning behavior, with differentiated predictions for teachers' experience. Reflection on experience and self-efficacy unexpectedly was not found to be relevant. Teachers-on-the-job were generally better at diagnosing their students' learning behavior than were preservice teachers without teaching experience. However, the diagnostic work of preservice teachers with teaching experience was as credible as that of the in-service teachers.