How to make sure that test cases have covered all functions? Is there any tool/software can be used to review test cases?

I have a question: "How to make sure if test cases covered all functions? Is there any tool/software used to review test cases?"

This is the methods that I'm using:

1. Read all related documents/requirements to understand about the functions
2. Create test cases manually or use Excel to create a matrix then write down test case one by one
3. Reviewed by self (execute test cases then update if there is any mistake/missing)
4. Reviewed by leader

However, It didn't completely fulfill the question given above.

Please help me answer it. Thanks!