I've been looking into some federation issues, and the lemmy-federate tool is great. Especially the errors, check the errored instances to find out why they are erroring.
I've found two instances where one of my communities wasn't federating because it was misconfigured and the community was blocked.
I've found two instances where the federation was working, but because I as a user was banned, the communities were empty.
And I found one instance that WAS federated by a user, then the user left the instance, and now the community is just dangling without updates.
https://lemmy-federate.com/
Thank you! I found it by googling it right after I posted that.