I’m no expert but my understanding is that you don’t, because that’s not exactly how it works.
I believe that you can see what instances it’s not federated with, but it basically federates with any instance where a kbin user has followed an element on that instance. However I think that still only feeds that part of the instance through to kbin, so it’s less ‘kbin federates with that instance and more ‘you can follow elements from any instance you want’
The federated public feed is basically your instance, and anything people from your instance are subscribed to.
You can also block instances privately, or choose to defederate on an individual basis, if you don’t wish to see content from other instances at all.