There are several free online tools available to detect what WordPress theme a site is using. Some of the most popular ones are:

1. What WordPress Theme Is That: This tool allows you to easily detect what WordPress theme a site uses (including parent and child themes), as well as what WordPress plugins are being used.

2. WordPress Theme Detector: This tool analyzes a site and displays a whole lot of information about the WordPress theme and plugins currently being used by the site.

3. Scan WP: This tool allows you to take any WordPress website and tell you what plugins and themes are being used by that website. It can detect the theme being used by simply adding the site in question to the form and clicking "detect".

4. Kinsta WordPress Theme Detector: This tool allows you to discover what theme is being used on a site, along with other key information about it, such as the theme name, URL, version, and author.

5. CodeinWP What WordPress Theme Is That: This tool is similar to the first one mentioned above, and can easily let you know what theme a site is using.

If you want to find out what WordPress theme a site is using, you can use any of these tools to detect it.