I believe a proper discovery session with the client, would help you understand what the client defines as "requirements".
Also, it's true that a lot of clients do not have a complete understanding of what they want their products to do, hence, a usability test is also advised, and if it meets their requirements, then that should be a go.
Finally, I also think it'll be a good move to carry your client along in your design process, so they're aware of what is, and what is not. That way, any iterations that need to be made, are made during the process, not after completed efforts.