Understanding how best to assess expertise, the situational variations of expertise, and distinctive qualities of expertise that arises from particular workplace experiences, presents an important challenge. Certainly, at this time, there is much interest in identifying standard occupational measures and competences, which are not well aligned with such variations and distinctiveness in performance. Therefore, this paper addresses the methodological issues posed by such a challenge through reviewing the expertise literature to identify ways forward. Based on the example of one target domain, medicine, and one criterion task, the comprehension of visualizations, the study identifies, elaborates, and evaluates assessments used to study qualitative changes in professional vision that occur as a result of extended periods of workplace learning experiences. It identifies the kinds of sample, materials, measures, and analysis methods used to assess the quality of expertise differences, as well as what elements of and differences in visual expertise are revealed in particular assessment processes. In doing this, the study seeks to illuminate how assessing the quality of expertise differences matured over the past 20 years, noting that strategies of scanning brain activity and tracking eye movements are now being used in ways that augment traditional approaches such as using verbal reports and observing representational practices. The findings demonstrate how the assessment of visual expertise has become more multidisciplinary over the past two decades. Implications for educational practice and future research directions on the development of professional vision are discussed.