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.
Yeah I updated my original post on lemmy.world. People used it so much that they exceeded the monthly bandwith limit of 100GB. I'm working on moving it to another host and optimizing the bandwith usage.