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.
Just to note if copying the URL you have to strip off the https:// or else it won't work (maybe people just don't copy that these days, but I ran in to that problem anyway).
Uh yeah that is true. This was a one afternoon project, so I kinda threw the frontend together without thinking too much of it. I guess truncating the protocol instead of relying on the users doing it makes sense.