I couldn't find any tools to check this, so I built one myself.
This is a little site I built: the Defederation Investigator defed.xyz. With it, you can get a comprehensive view of which instances have blocked yours, as well as which ones you are federated with.
The tool is open source and available on GitHub. Hopefully someone will find it useful, enjoy.
Register your interest here if you'd like to join a team of smaller instance admins backing each other up. So if it's the middle of the night or we're on vacation, someone we trust from the federation of small instance administrators can secure our sites if we're unavailable.