Go to the bottom of any page and there by the link to the modlog is a link “Instances”. That’s a long list of all instances that server is federated or defederated with. (It’s actually kind of disturbing how many federated instances there are that don’t seem to be active - possibly just slurping data? But that’s a different story).
Anyway, scroll down and make sure the whole page loaded, then search for the name of the one you’re looking for in your browser, and see if it’s on the federated or defederated list (Linked Instances vs. Blocked Instances).