I believe the following function will solve your problem:
Code:
Function CanSaveDocAsTxt() As Boolean
Dim i As Integer
With ActiveDocument
For i = 1 To .Characters.Count
.Characters(i).Select
'if AscW(.characters(i).Text) > 127 then exit function
With Dialogs(wdDialogInsertSymbol)
If .Font <> "(normal text)" Then Exit Function
End With
Next i
End With
CanSaveDocAsTxt = True
End Function
It goes through all characters in the active document and returns True if all characters in the document are part of (normal text) character set. Bear in mind that this character set has ANSI characters (character code points between 128 and 255). If you want to limit your text file to characters in ASCII range (0-127), then uncomment the commented statement above. This function does not check the document for shapes (images, etc.) - But I assume you know which documents do!