Typically way of assessing the completeness of test cases is “Requirement Traceability” where in test cases is mapped to each requirement to ensure that each requirement does indeed have test cases.
Is this logic strong enough? Unfortunately NO! Why? Assume that each requirement had just one test case. This implies that we have good RTM i.e. each requirement has been covered. What we do know is that could there additional test cases for some of the requirements? So RTM is a necessary condition but NOT a sufficient condition.
So, what does it take to be sufficient? If we had a clear notion of types of defects that could affect the customer experience and then mapped these to test cases, we have Fault Traceability .This allows us to be sure that our test cases can indeed detect those defects that will impact customer experience. Combining both Requirement traceability and Fault traceability will help us to judge the sufficiency of test cases.