IsMasterDocument Property
True if the specified document is a master document. A master document includes one or more subdocuments. Read-only Boolean.
Example
If the active document is a master document, this example switches to master document view and opens the first subdocument.
If ActiveDocument.IsMasterDocument = True Then
ActiveDocument.ActiveWindow.View.Type = wdMasterView
ActiveDocument.Subdocuments(1).Open
Else
MsgBox "This document is not a master document."
End If