the nature of federation/decentralization makes it a bit hard to get a central list. some instances publicly list which other instances they blocked and which they federate list. but other than checking each instance one by one, I'm not sure there's a way to know.
You can check for any instance who they have federated with and who they have blocked by navigating to their /instances page.
For example for Beehaw: https://beehaw.org/instances
I think it just means their server is having some trouble serving requests 😅
That's a public URL on Beehaw side, so whenever you open that URL, they'll actually have no way of knowing what your home instance is what its federation status is.
I'm not sure if it's exactly the same thing, but you can click on Instances at the very bottom of the instance, or add "/instances" to the url e.g. https://lemmy.ml/instances to see the list of blocked instances.