One way to tell if a website is using WordPress is to look for the "wp-content" directory in the site's source code. WordPress sites typically have a "wp-content" directory that contains the site's themes and plugins. You can view the source code of a web page by right-clicking on the page and selecting "View page source" (or a similar option) from the menu. This will open the page's source code in a new tab or window, where you can search for the "wp-content" directory.
Another way to tell if a website is using WordPress is to look for telltale signs in the site's URL structure. By default, WordPress creates permalinks (i.e., URLs) for posts and pages in the form of "http://example.com/sample-post". This URL structure is unique to WordPress and can be a good indication that a site is using WordPress. However, it is worth noting that this is not a foolproof method, as some WordPress users may choose to use a custom URL structure.
