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.
Definitely, but I can see what the TC is saying. This tool simplifies the process even further for troll(s) who campaign against those trying to evade them.
On mastodon, in my circles at least, it's taboo to link to the equivalent for mastodon. It makes any harassment campaigning, trolling and re-routing much easier. Unless you're willing to build the tool yourself, you just don't really know these numbers. Once you use the tool (and I have seen and used it), you see very easily who the big defederators etc are.
I'm going to bet that this tool is not worth what ever value or curiosity it provides. It so easily puts targets on instances backs, including smaller instances that are probably easy targets.
I think it's probably the most important piece of information to have when choosing a server.
While a lot of people have said "it can be used for harassment", I've not seen any information yet on how that could be. On the contrary, I think being open and up-front about federation policies is important for a healthy community - it avoids arguments about going behind people's backs.