Scenario: In previous versions of Word, you may have been able to access the View.SplitSpecial property to verify if the Reviewing Pane is open. However, in Word 2013, View.SplitSpecial is value 0 whether the Reviewing Pane is open or closed. Additionally, View.SplitSpecial can still be set to the value wdPaneRevisions through the Object Model, which will continue to open the Reviewing Pane.

Cause: Prior to Word 2013, another pane could not be opened at the same time as the Reviewing Pane. However, the behavior was changed such that Reviewing Pane can be opened in addition to another pane (this is similar to how the Navigation Pane works). At time of writingthe wdPaneRevisions value will not be returned regardless of state.

Solution: A workaround for this scenario in Word 2013 is to access the Revisions index in the CommandBars object, and check the Visible property to determine if the Revisions pane is open (True means it is open, False means it is closed). An example of this in VBA can be seen as such:

Sub AlertRevisionStatus()
 If CommandBars("Revisions").Visible = True Then
  MsgBox("Reviewing Pane is open.")
  MsgBox("Reviewing Pane is closed.")
 End If
End Sub