I recently discovered that Google had indexed some of my subdomains (for example, subdomain.domain.com) used for testing and when migrating data from Joomla to WordPress.
It came as quite a surprise that Google found these subdomains, and quickly needed to know what other subdomains Google had in its index so I could 301 redirect the traffic.
This let me to the following Google search, which displays all the subdomain content indexed for any given domain.
Simply replace ‘mockbox.net’ with the domain you’re searching.
The results will include all the indexed content that came from a subdomain of your main website domain.