Use the NewValue property on the TField when reading the second record (the one which contains the edits). If the returned variant is empty (or Unassigned) then that field was not modified. Here is some code that demonstrates this:

   var
     NewVal: Variant;
   begin
     NewVal := DataSet.FieldByName('MyStrField').NewValue;
     if VarIsEmpty(NewVal) then
        ShowMessage('Field was not edited')
     else if VarIsNull(NewVal) then
        ShowMessage('Field was blanked out')
     else
        ShowMessage('New Field Value: ' + String(NewVal));
   end;