Document.IsMasterDocument (Word)

True if the specified document is a master document.

A master document includes one or more subdocuments.

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