Serial entrepreneur and founder as Satori, Trip.Center, Spotless and other upstarts. Possess experience in idea development, product development, online marketing, finance.
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.