You need to use a watchpoint. From the page:
- Highlight the variable in the editor, or select it in the Outline view.
- Click Run > Toggle Watchpoint.
- Do any of the following:
- To stop execution when the watch expression is read, select the Read check box.
- To stop execution when the watch expression is written to, select the Write check box.
- The watchpoint appears in the Breakpoints view list.