Mastodon

On desktop there should be a small "about" link in the bottom left, from which there is a "moderated servers" dropdown menu where you can see defederated servers. I don't see anywhere to view the servers you ARE federated with, but if it's listed at ~~Mastodon's official website~~ (whoops, I actually meant this)it's probably in.

I don't see anything on the official Masto app, but that app is just missing a ton of functionality in general.