Diagnostic assessment can help you identify your students’ current knowledge of a subject, their skill sets and capabilities, and to clarify misconceptions before teaching takes place (Just Science Now!, n.d.). Knowing students’ strengths and weaknesses can help you better plan what to teach and how to teach it.

Types of Diagnostic Assessments

Pre-tests (on content and abilities)
Self-assessments (identifying skills and competencies)
Discussion board responses (on content-specific prompts)
Interviews (brief, private, 10-minute interview of each student)

Reference: https://www.niu.edu/citl/resources/guides/instructional-guide/formative-and-summative-assessment.shtml