I get dozens of messages a day and have studied my list of messages every day and still miss some occasionally. It would be a bit easier if I only got 10 to 20 a day but I get much more and cannot tell which I have answered. They do not stand out in any way.
This is a real problem and seems it would have an easy solution. I do not see the little clock symbol any more on the unanswered messages like I used to so there is no way to tell which I still need to answer.