Rapid Cache will add "HTML Notes" to the bottom of every page when it Rapid Cache enabled and you have the HTML Notes option enabled (enabled by default). To verify that Rapid Cache is enabled and the HTML Notes option is turned on, visit Rapid Cache → Plugin Options → Enable/Disable:

Once you have verified that Rapid Cache is enabled and the HTML Notes option is turned on, you can logout of WordPress (caching is disabled for logged-in users by default, so you'll need to logout) and then visit the home page of your site and choose the "View Source" option in your browser:

• Internet Explorer: View → Source
• Google Chrome: View → Developer → View Source
• Mozilla Firefox: Tools → Web Developer → Page Source
• Apple Safari: Visit Safari → Preferences → Advanced and enable "Show Developer menu in menu bar". Then you can visit Developer → Show Page Source

You'll want to scroll all the way to the bottom of the source and look for the HTML Notes added by Rapid Cache. They should look something like this:

If you see "Rapid Cache fully functional", then you know Rapid Cache is enabled and working properly.

If you don't see any HTML Notes added by Rapid Cache but you do see several empty lines where you'd expect to the see the notes, then you could have another plugin or another service stripping out HTML Notes from your site. CloudFlare is known to do this when the "Minify" option is enabled, for example.

This does not mean that Rapid Cache isn't working. It just means that the HTML Notes that Rapid Cache adds for informational purposes are being stripped out. If you want to verify that Rapid Cache is actually working, you can temporarily disable any other "HTML Compression" or "Minify" options that you're running.

Note: If you're using Rapid Cache Pro with the HTML Compression option, Rapid Cache will not strip out its own HTML Notes.

If Rapid Cache is enabled but does not appear to be working, here are a few things you can check:

• Try clicking the "Restore" button inside the Rapid Cache options to reset the Rapid Cache configuration. Note that this restores all of the default options, so if you have customized your Rapid Cache configuration you should back it up before doing this (if you have Rapid Cache Pro, you can backup your options in Rapid Cache → Plugin Options → Import/Export Options).
• Make sure there are no other WordPress caching plugins enabled. Only one WordPress caching plugin can be running at a time.
• Check your .htaccess file for code that may have been added by other caching plugins. Some caching plugins, such as WP Super Cache and W3TC do not "clean up" after themselves when uninstalled and will leave behind code inside the .htaccess file that can cause issues if you try using another caching plugin.
• The wp-config.php file should have a line at the top that says define('WP_CACHE', TRUE);--there should be no other lines that mention WP_CACHE inside the wp-config.php file, only that one line at the top, right below <?php. This is what enables caching on a WordPress site. If WP_CACHE is set to true, then WordPress will load the caching system defined inside the wp-content/advanced-cache.php file (see next point).
• The wp-content/advanced-cache.php file should contain PHP code that mentions Rapid Cache at the top of the file. This is the file that all WordPress caching plugins use to "hook into" WordPress, so if you installed another caching plugin on your site the advanced-cache.php file may be missing, corrupt, or contain code from another caching plugin. Rapid Cache tries to ensure this cache file exists and contains the right content, but in some scenarios it may be necessary to deactivate Rapid Cache, delete the advanced-cache.php file, and then reactivate Rapid Cache so that it can reinstall the file with the correct code inside.