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.
IF YOUR WAIFU WAS REAL SHE WOULDN'T LIKE YOU. :blobcatsipglare: YOU'RE A SUCKER. KILL YOURSELF, BECAUSE NO HITMAN SHOULD EVER HAVE THE DISPLEASURE OF INTERACTING WITH YOU. :uniinsane:.