The following code will detect /most/ of the mergefields in a document - it
may not see some of the fields in the graphics layer.

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

If you take the name of the merge field and use it as the index to the

ActiveDocument.MailMerge.DataSource.DataFields()

collection, an error should be raised if the field is not in the collection
(I haven't checked to ensure it is trappale - if not, you would have to
iterate through the collction and try to match each fieldname in the data
source).

However, as a further complication, Word may alter the data source field
names when it inserts them as MERGEFIELD fields, if
a. they contain certain characters
b. they are over a certain length (over around 31 characters I think)

I don't have a list of the relevant rules or code that deals with them. I
suspect merge documents where renaming occurs are relatively rare.

Peter Jamieson