I found a solution, thanks for @me how.
I go through word by word in the Active Document and then if Font.Properties are set to Hidden then show a Message Box.
Here is the code:
Sub ToggleShowHiddenText()
For Each sentence In ActiveDocument.StoryRanges
For Each w In sentence.Words
If w.Font.Hidden = True Then
MsgBox w
End If
Next
Next
End Sub