Skill-based outcomes play a prominent role in citizen science experiences as they are situated directly between the dual, intertwining goals of advancing science learning and science research. Regular and transparent assessment of volunteers' science inquiry skills can support these goals, but we lack a comprehensive understanding of how these skills are assessed. To address this gap, we collected data from citizen science project leaders through questionnaires, interviews and a systematic literature review. Findings demonstrated that most projects targeted a narrow range of skills that centered on data collection. About half of questionnaire and interview respondents stated that they assess science inquiry skills, but many relied on informal or indirect methods such as observations and conversations lacking protocols. Less than a third of these respondents and only 13% of reviewed articles administered formal assessments of skills, such as tests and performance measures based on volunteers' submitted data. Challenges associated with skill assessment included lack of time, staff, expertise, funding and supporting resources. Overall, our study provides clear evidence of the limited breadth and assessment of skills targeted within citizen science, and points to the need for resources that promote and expand the use of assessment addressing science inquiry skills.