How can users check to see if their instance is defederated with any other instances?

I haven't seen a way to check on kbin, yet. But if you suspect a Lemmy instance has blocked you, you can check lemmys block list at, for example, lemmy.world/instances.

Came here to ask the same question.

I don't know if this was a joke or not but according to this user kbin doesn't defederate?

Not implemented yet. Ernest is working on moderation tools, though.