You can use the Windows EM_LINEFROMCHAR message to get the current (zero based) line number (position of the caret) of any edit control. Example:

procedure TForm .Button Click(Sender: TObject);
var
  LineNumber : integer;
begin
  LineNumber := SendMessage(Memo .Handle,
                            EM_LINEFROMCHAR,
                            word(- ),
                            0);
  ShowMessage(IntToStr(LineNumber));
end;