How to evaluate that the Test Cases covers all the Functionality?

Answer 1

Whatever you are working on, it is probably part of a development methodology. If the methodology is sound, there should be a function requirement documentation or product requirement specification. Different methodologies adopt different names but the idea is the same; that the required specifications/features are documented.

To check that the test cases are fully covered, one would have to look at every single requirement that is spelled out in the requirement document and see that it is represented in the test plan/test case documentation. If there are new features introduced into the product during development, these features need to be updated into the requirement documentation.