All links to external files in the Excel document contain square brackets. So if you want to find a reference an external file in the Excel document you need look for the square brackets.
Open the Excel document. Press the “Find&Select” button in the “Home” tab. Select the “Find” option in the drop-down menu.
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In the pop-up window, type the square bracket symbol in the “Find what” box, then press “Options” button.
In the appeared options choose to look “Within” the “Workbook”.
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Press “Find All” button.
All the external links in the document have been successfully found.