function FileINUse(filename:String):boolean;
var
  usein:file;
begin
  result:=false;
  assignfile(usein,FileName);
  try
    reset(Usein);
  except
    on E:EInOutError do
    begin
      if e.ErrorCode=32 then
      begin
        result:=true;
        exit;
      end;
    end;
  end;
  closefile(usein);
end;

procedure TForm1.Button1Click(Sender: TObject);
  if OpenDialog1.Execute then
  begin
    if fileInuse(OpenDialog1.filename) then
      ShowMessage('文件正在使用中')
    else
      ShowMessage('沒(méi)有使用');
  end;