[EN] In recent times, education for sustainability is gaining more and more relevance in the field of engineering. One of the recently established Sustainable Development Goals explicitly calls for the achievement of sustainable infrastructures by 2030. Infrastructures are recognised as a crucial element towards a sustainable future, as they are responsible for a vast amount of environmental damages and economic expenses, but also are considered to be essential to achieve social welfare and the economic development of regions. Sustainable design of infrastructures requires a paradigm shift in the way engineering students are taught how to face design problems. The complex relation that exists between the conflicting dimensions in which sustainability is founded, namely economy, environment and society, requires of future engineers to develop a series of transversal competencies during their postgraduate studies. However, there are no objective criteria on which to base the assessment of the degree of acquisition of these skills. This paper presents an objective methodology for the evaluation of one of the key competencies required for sustainability-oriented future engineers, namely critical thinking. The proposed methodology is based on the evaluation of the coherency expressed by the students when dealing with a sustainability-related design problem. Such consistency shall be mathematically quantified by means of the Analytic Hierarchy Process. Through the resolution of a particular case study, where students are required to complete a survey comparing different sustainability criteria pairwise, the professor is able to obtain a consistency index from the comparison matrices derived from the survey results of each student. Such indicator is a valuable tool to assess both the clarity with which students perceive sustainability as well as their ability to meet the new challenges that the society will face in the near future. The authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness, along with FEDER funding (Project: BIA2017-85098-R).