Edit Comments in Excel

I Have simple question. I’ve developed a simple spread sheet application. The sheet is protected. I want to use a single button to create a comment within a selected cell and to edit the comment when required. In order for the simple macro to work, it appears I need to identify if the selected cell already has a comment. If so I cannot use “AddComment”. The Help file does not give any indication how this would be done in a simple, efficient manner.