The article is devoted to an actual problem of Russian education - the diagnostics of skills and qualities of students to reason and argue in training skilled workers to work in conditions of technological manufacturing or service sectors. High requirements to personal qualities of the modern worker are explained by the needs of modern society and high competition. The special value is currently owned by the skills and qualities of business communication and teamwork, of evidence and convincing arguments, making reasoned non-trivial solutions in work situations. On this basis, reasonably-reasoned skills and qualities can be considered systemically important component in the system of the individual competencies development. The effectiveness of the directional formation of skills and qualities to reason and to argue requires measuring materials and diagnostic techniques. Conceptual bases of study skills and qualities have established criteria for measurements and process data obtained by methods of mathematical statistics. Diagnostic results make it possible to monitor the dynamics of development, compare different in the number of students and the areas of training study groups, to assess the effectiveness of implemented educational technologies and techniques. As a result, proven diagnostics, used in the study, established a connection between the development of sensible and argue skills and qualities, with one hand, and the formation of general and professional competencies of the students, on the other hand. he study relates to the internal procedure following criteria: level of achievement; level of abstraction; level of awareness of learning; level of development of figurative and spatial concepts; analytical quality when working with professional information; and the following abilities: to establish causal relationships and dependencies; highlight the main, to formulate conclusions; to take a more reasoned point of view of a friend; to put questions, proceeding from logic of the business situation; to plan and implement activities according to the algorithm. The study relates to external procedural criteria emotional, volitional qualities, independence and quality of discreet business communication. Scoring criteria include: the kind of cognitive activity, the activity, the level of the operating basis of argumentation, the quality of rhetoric, general and professional competences.