Document.VBASigned (Word)

True if the Microsoft Visual Basic for Applications (VBA) project for the specified document has been digitally signed.

Documents.Open FileName:="C:\My Documents\Temp.doc" 
If ActiveDocument.VBASigned = False Then 
 MsgBox "Warning! This document " _ 
 & "has not been digitally signed.", _ 
 vbCritical, "Digital Signature Warning" 
End If