How can I determine if the results on Special:Search are returned by Cirrus and not standard MediaWiki?
Determine the search backend used
Either append '&cirrusDumpQuery' to the search URL for a data-dump in json-format or alternatively use a cirrus specific search keyword, e.g.
If it's only for debugging purpose appending '&cirrusDumpQuery' to the Special:Search URL should dump a json containing the elasticsearch query if CirrusSearch is being used. If adding such param does not change the output then the standard MW search (or another search engine) is being used. If it's not for debugging purpose then I doubt there is an obvious way to determine this from the UI.
You could also use some cirrus specific keywords and see if they work: e.g.
Perfect. This really helped. Indeed this was to check after an upgrade if things are still working and not for continuous monitoring. There was an issue for the interaction with another extension which caused Cirrus to fail initially. All of these worries: gone.
Both ways to determine the backend are great. Admittedly I could have come up with the second option too. In the heat of the action ... :)