If you're comfortable with going through the page's source code (https://www.computerhope.com/issues/ch000746.htm), you can use the following telltale signs to determine the CMS currently in use:
1. Search for the "generator" meta tag, it contains exactly the information you need.
2. Search for CMS-specific directories, such as "wp-content" for WordPress or "templates" for Joomla.
3. Apart from checking the code, you can also try visiting specific URLs, e.g. try adding "/administrator/" to the homepage URL to see if it's Joomla or "/user/" to see if it's Drupal.
Please note, however, that any of those can be changed manually by the site admins, so there's no 100% precise method, to the best of my knowledge, unless you decide to simply ask the site admins :)
You can also install a Chrome extension such as the Wappalyzer which will tell you a page's underlying CMS automatically.
If you're on a budget, I strongly recommend using WordPress, as it has the steepest learning curve and offers a much wider template and plugin choice. You can determine which theme a particular website is using e.g. here: http://satoristudio.net/what-wordpress-theme/, the tool will give you a direct link to the (parent) theme's official web page - or let you know that the theme has been modified too far to be recognizable.