How do I track down 'invalid references'

I have a workbook that has many worksheets and associated formulas. Every
once in a while when I close the file I get the following message:
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A formula in this worksheet contains one or more invalid references.
Verify that your formulas contain valid path, workbook, range name, and cell
reference.
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I've been able to track this checking to be after "Workbook_BeforeClose"
execution, but I don't have a clue on finding the formulas with the invalid
references.
I found cells with "#DIV/0!", "#VALUE!", #NUM!, #N/A.
Which of these should I attach first? Could it be something else?

Any suggestions?