As a Lemmy user, I believe the way to tell is when looking at the top, you should see the user@instance, and it is posted to "lemmy.world@lemmy.world", this community is the home of the lemmy.world instance so it's abit confusing, but say "gaming@kbin.social" is how it should show for Lemmy users
Edit: and speaking of the "first time I've seen a federated post", I heard kbin.social turned off federation since it was getting overloaded with users, much less other instances. It'll slowly get better