Co-design is fundamental to humanitarian engineering and increasingly recognised as such in engineering curricula. However, it is challenging to teach, learn, and assess. In this paper, we describe the development and validation of a scenario-based instrument to distinguish novice and expert approaches to co-design in the context of humanitarian engineering. The instrument assesses the extent to which respondents describe stakeholder participation in each of the "scope," "design," and "deliver" phases of the design process, with co-design experts taking a collaborative approach throughout. We analyse and compare responses to the instrument from first-year undergraduate engineering students and experienced humanitarian engineering practitioners. Implications for educators, to use this scenario-based assessment in their own research, teaching, and curriculum development, are discussed in detail.