You could ask somebody to visit the site and investigate it You could email the admin of the site You could look at the source code You could register at the site and find some clues You could look in the footer of the site for links and clues of template or designer Maybe these are too obvious, but they are some ways "how".
If JoyGoRound's tips don't help, perhaps try this site: http://www.quarkbase.com/ (free online service). It can sometimes identify what you need. I've found it okay for research -- it gives better info on some sites than others. Worth a visit! Kind regards,
you can check the source code. also check for the admin control panel. example: domain.com/wp-admin (wordpress) domain.com/administrator (joomla) domain.com/admin (drupal)
Sometimes you can tell by the robots.txt file, because the owner will exclude certain software folders from search engine spidering.
Generally speaking it's possible to program the site in the way that no one could determine what CMS has been used to edit it. Usually CMS is installed to its default location so by checking if it exists you can determine the brand.