You could start with something like the following, which inspects /all/
field types. I wouldn't guarantee that it will find them all but it's the
nearest I've ever got.

Sub IterateMergeFields()
Dim r As Range
Dim f As Field
Dim s As Shape
For Each r In ActiveDocument.StoryRanges
 If r.StoryType <> wdTextFrameStory Then
   For Each f In r.Fields
     If f.Type = wdFieldMergeField Then
       ' do whatever you need
     End If
   Next
 End If
Next
For Each s In ActiveDocument.Shapes
 If s.Type = msoTextBox Then
   For Each f In s.TextFrame.TextRange.Fields
     If f.Type = wdFieldMergeField Then
       ' do whatever you need
     End If
   Next
 End If
Next
End Sub

--
Peter Jamieson - Word MVP
Word MVP web site http://www.mvps.org/word