To find a text string in the comments with no case sensitive, please check the post here; if you need to find a text string in the comments with case sensitive, please follow the steps below for details:
Step 1: Click anywhere in the worksheet;
Step 2: Click the "Home" tab from the ribbon;
Step 3: Click "Find & Select", and select "Find" from the drop-down list;
Alternatively, please use shortcut Ctrl+F to bring up "Find and Replace" window.
Step 4: In the "Find and Replace" window, type the text string (or any letter or number) you want to find in the worksheet;
Step 5: Click "Options" in the "Find and Replace" window, select "Comments" in the "Look in" box; check "Match case" box in the middle.
Step 6: Click "Find Next" at the bottom to find the next record; or click "Find All" to find all the comments that contain the text string with case sensitive.