Shelly asked the question some time back. She wanted to know how could a non-technical person find out on what applications a website is running. The simplest answer would be to use a browser extension in order to find out. One such browser extension is Wappalyzer. It’s available for Firefox, Chrome and there is a tweak for Opera as well.
I no more recall since when I use Wappalyzer, been a long time. Wappalyzer results are accurate and would help anyone identify applications on a website without employing much technical skills.
Once enabled in the browser, it appears as an application icon in the right-end of the address bar. E.g, if the website runs on Drupal, the Drupal icon would be shown. You click on the icon and a drop-down menu appears showing more information. Wappalyzer would list the CMS type/version, libraries, webserver, operating system etc.
For the adventurous, there is the Nmap tool which can be extended using Nmap Scripting Engine (NSE).
Analyzing the webpage source also gives an indication on the CMS version and libraries used; in case the website runs on a CMS.