This is all you need to do... :)
1   
2   procedure TForm1.TreeView1DragOver(Sender, Source: TObject; X, Y: Integer;
3     State: TDragState; var Accept: Boolean);
4   var
5     tn: TTreeNode;
6   begin
7     tn := TreeView1.GetNodeAt(X, Y);
8     if Assigned(tn) then
9       Memo1.Lines.Add(tn.Text);
10  end;