In an era of urgent need of sustainability, initiatives aimed at developing competences for sustainability need assessment tools to effectively support understanding on their impact. This work presents the creation process of a questionnaire to assess the GreenComp framework competences “Valuing Sustainability”, “Supporting fairness”, and “Promoting nature”. After a literature review on research about sustainability competencies and its data collection tools, it was developed a questionnaire using a mixed-methods study design concerning three phases. From this research resulted a versatile tool that has been specifically crafted for extensive use in the realm of sustainability education research, although easily applied in a wide range of research, educational, and training contexts.