Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function IsFileInUse(const fName: TFileName): Boolean;
 var
   HFileRes: HFILE;
 begin
   Result := False;
   HFileRes := CreateFile(PChar(fName),
                          GENERIC_READ or GENERIC_WRITE,
                          0,
                          nil,
                          OPEN_EXISTING,
                          FILE_ATTRIBUTE_NORMAL,
                          0);
   Result := (HFileRes = INVALID_HANDLE_VALUE);
   if not Result then
     CloseHandle(HFileRes);
 end;


 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if OpenDialog1.Execute then
     if IsFileInUse(Opendialog1.FileName) then
       ShowMessage('File is in use!');
 end;