Most sites leave clues about what they are built on. A few checks will identify the majority of them.
Look at the page source
Right click, View Page Source, and search for a generator tag. Many systems announce themselves there:
<meta name="generator" content="WordPress 6.5" />It is often removed for security, so its absence tells you little.
Look at the file paths
The paths to stylesheets and images are a stronger signal, because they are hard to hide.
/wp-content/ or /wp-includes/ | WordPress |
/media/jui/ or /templates/ | Joomla |
/sites/default/files/ | Drupal |
/typo3conf/ | TYPO3 |
static.parastorage.com | Wix |
static1.squarespace.com | Squarespace |
Look at the URLs
Login paths are habitual. /wp-login.php is WordPress. /administrator/ is Joomla. /user/login is Drupal. Hosted builders normally log in on their own domain instead.
When nothing shows up
Two situations defeat all of this. A headless setup separates the editing system from the public site entirely, so the front end reveals nothing about what is behind it. And a site behind a proxy such as Cloudflare will hide some server headers. In both cases you may simply not be able to tell from the outside.