Critical thinking is a fundamental skill for 21st-century citizens, and it should be promoted from elementary school and developed in computing education. However, assessing the development of critical thinking in educational contexts presents unique challenges. In this study, a systematic mapping was carried out to investigate how to assess the development of critical thinking, or some of its skills, in K-12 computing teaching. The results indicate that primary studies on the development of critical thinking in K-12 computing education are concentrated in Asian countries, mainly focusing on teaching concepts such as algorithms and programming. Moreover, the studies do not present a fixed set of critical thinking skills assessed, and the skills are selected according to specific teaching and research needs. Most of the studies adopted student self-assessment using instruments that are well-known in the literature for assessing critical thinking. Many studies measured the quality of instruments for their research, obtaining favorable results and demonstrating consistency. However, the research points to a need for more diversity in assessment methods beyond student self-assessment. The findings suggest a need for more comprehensive and diverse critical thinking assessments in K-12 computing education, covering different educational stages and computing education concepts. This research aims to guide educators and researchers in developing more effective critical thinking assessments for K-12 computing education.