You may get ideas from a research guide or an article you have found. Explore them and find examples:

A careful examination of the organization and contents notes mentioned in the "Collected Works: organization"  section of this LibGuide can also help you identify what may be omitted.

Compare the contents notes to the works lists in the Grove Music Online at Grove Music Online to verify what works might be missing. Note: For many composers the works lists are annotated with corresponding collected works volumes. You can use the works lists to examine overall coverage in collected works editions.