At this time, Google Apps Script cannot do exactly what you want:
access a comment for a specific cell of your choosing. It also cannot determine what cell a comment is attached to.
You can see question 15556476 for additional information. Even better, please go star Google's Issue 36756650, and hopefully that will prompt them to release this functionality we all want.
There aren't any references to Comments within the current API for Spreadsheet ranges. And, in the Drive Comments API, the "anchor" property does not work with Spreadsheets. (Also see question 56544407.)