WordPress, Joomla, and Drupal are primarily powered by PHP.
And it's quite easy. Let's get started.
View Page Source
Go to the outer edge of a site (e.g. far left), then right-click and click on "View Page Source" (or the option that's similarly named).
You can now read through it and see what that page is created with.
You can also use Ctrl + F (command + F in Mac) to search through it for certain key words.
If you want to drill down on a specific part of a page, most browsers now come with built-in inspectors. Or you can use Firebug in Firefox.
Just right-click on the object you'd like to inspect, then click on Inspect Element (or the option that's similarly named).
The respective code will be high-lighted in the left window. Then you can click on the tags outside of it or within it to drill down on those. As you do that, it will high-light the part of the page it controls.
On the right side, you'll see the CSS styles that are applied to the HTML tag that's high-lighted.
This is an extremely useful and quick way to inspect code.
If you just want a quick way to see which technologies a website uses without doing the manual investigation, then check out our Wappalyzer tutorial.