the WordPress Plugin checker can find which plugins are installed on almost any WordPress site. Unfortunately, this tool doesn’t list all of the plugins it detects. Instead, it looks to see if any of the plugins developed by Earth People are installed along with the 50 most popular plugins.
Despite having a blank index.php file in place to prevent visitors from seeing a directory listing of the plugins folder, the plugin checker was able to determine that the site was using five of the most popular plugins.

WordPress Plugin checker