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.
Since I was using this to write a little defederation thing on which instances find hexbear so repulsive they had to defederate I noticed the number from exploding-heads go up quite a bit from the 75 to 170 now. I will make it a point to call out two specific instances for being hilariously disgusting feddit.de aka the german federation, and feddit.dk both defederated hexbear but didn't defed the outright nazi instance exploding-heads earlier today, class act from the german and danish mod team.
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).
I couldn't find any tools to check this, so I built one myself.
Literally the first link on join-lemmy.org has a table showing all instances and it shows both how many instances that instance is blocking "and* how many other instances are blocking that instance