Check for field in tabledefs prior to append

The following code works well except I can't think of an elegant way
of checking for the existance of the named field prior to creating
it.  Suggestions?

Public Sub pNewField()
'Add a new field to every table in the collection
Dim strSQL As String
Dim db As Database, tdf As TableDef, intI As Integer

    Set db = CurrentDb

    For intI = 0 To db.TableDefs.Count - 1
        Set tdf = db.TableDefs(intI)
        ' Skip system tables
        If Left(tdf.Name, 4) <> "MSys" Then
            tdf.Fields.Append tdf.CreateField("Source_tbl", dbText,
15)
            strSQL = "UPDATE [" & tdf.Name & "] SET Source_tbl = " _
                & "'" & tdf.Name & "';"
            DoCmd.SetWarnings False
                DoCmd.RunSQL strSQL
            DoCmd.SetWarnings True
        End If
     Next

End Sub

Thanks,

Tim Mills-Groninger