What? Nearly every UI for the fediverse displays the instance of the poster after their name if they are not from the same instance as you.

That's why to you I show up as wizardbeard@lemmy.dbzer0.com, and to me you show up as Patnou@lemmy.world

There's no need for some sort of icon, you show your home instance wherever you go already.

Would the handle not suffice? The @lemmy.world part of every user and community of the instance.

Or, if that's what you are asking for, locally Lemmy doesn't show a handle, and looking at the settings of an instance I have an alt on, there doesn't seem to be an option to toggle the display of handles, for either local or external users/communities. In that case, using a function on that idea on Mbin, I'd second the idea of implementing on Lemmy.