For instance, this one (link to a post to !memes@reddthat.com): https://reddthat.com/post/20260613
Pasting it in your search bar should give you this kind of results:
You can then click on it to access the post from your instance (in this example, lemmy.zip: https://lemmy.zip/post/16918691)
Hm. So I understand the different comment IDs on different servers – the point is that searching for https://discuss.online/comment/9004867 on lemmy.world should then return a link to that same comment, with the right ID number for lemmy.world, on lemmy.world. Because through its federation backend it’s able to fetch the comment in question from discuss.online and then determine what is the local ID number on lemmy.world. Exactly the same as how it works for posts.
I just mucked around with it, and it works sometimes but not other times. I suspect that it’s because of backed up federation queues or too-short timeouts or something, but it definitely works some of the time. If it’s unreliable it may not be that good an idea to put into practice, though, of course.
Here is an excellent comment from 3 days ago, so it should be federated everywhere by now? I just shared that link with someone else in fact, in an unrelated reply. I then went to Lemmy.World and searched for that link, but it does not find the original, although it does find my recent sharing of it. I also searched on reddthat.com, and saw the same behavior. And lemm.ee as well. So the “sometimes it works” effect may have some additional not-entirely-characterized triggers for that behavior, but it definitely does not work all the time as a “find this comment” feature as we are discussing.
So as it now stands there is a way to use this search feature to find posts, but not reliably to find comments. Though that is still more than I knew a week ago before Blaze shared this trick with me then:-).