WPdetector is described as 'free tool that helps users find out the theme and plugins used by a WordPress website. In short, it is a theme detector and a wp plugin detector. WP Detector also shows additional information about the website and its domain'. There are five alternatives to WPdetector, not only websites but also apps for Wordpress. The best alternative is WPThemeDetector, which is free. Other great sites and apps similar to WPdetector are Themechi (Free), What Wordpress Theme Is That? (Free), whoiswp (Free) and Satori WordPress Theme Detector (Free).
What WordPress Theme Is That is a free online tool allows you to easily detect what WordPress theme a site uses (including parent and child themes). It will also detect what WordPress Plugins are being used.