There are more and more Lemmy instances (servers) every day sooooo this isn't a terrifically comprehensive search but, if you're looking for something in a federated Lemmy instance, one of the Lemmy.* instances probably knows about it... Maybe that's enough?
In other words, I would hazard a guess that almost everything in the lemmyverse can probably be found by searching the Lemmy.* instances.. assuming they expose their content to Google in the first place.
It looks promising, I searched for something using site:lemmy.* and one of the results was on sh.itjust.works and was found because of the federated URL: