Your mention of plugins made me think if there’s a way to check the version information, and I found this topic that describes how to get the version information.
So I can see, for example, that this instance is using commit 276bc8a565389ea1a145af08ec8e64c1a5bea990, the FSF member forum is using commit 7ecaf6295daf8759aa98d00e7035c4dc0f853303, the Purism forum is using 999aaa35a79ae7c586a91de1f6c1f7b3c8092bd2, and the Exercism forum is using commit 1bd9ca11e777f880462ae64c6795ef7de28a8cd2. All of those are valid commits in the official repo, so I guess the instances have not modified Discourse itself (though they may have added plugins).
I think that leaves plugins and themes. It looks like maybe you can see the name of the plugins by looking for “data-discourse-entrypoint” attributes in plugin scripts, but I don’t see a version identifier. Do you know of any way to get the list of installed plugins?