Complex systems understanding is crucial for understanding the sciences of the twenty-first century. Helping students with the principles of complex systems is thus both a challenge and an opportunity. In order to investigate the effectiveness of complex systems learning activities, we would like to have an assessment instrument which we can ultimately use as pre and post-test to assess the learning gain. In this paper, we developed an assessment instrument as a questionnaire containing five scenarios related to key concepts of complex systems understanding. After validating the understandability of the questions, we collected answers from 37 participants including experts as well as non-experts. We finally propose a scoring scheme based on fuzzy logic. This approach results in a general procedure which is compact, automatic and human legible to quantify expertise in complex systems concepts.