Magento2: How can you check/set which Elasticsearch index is currently being used

We have a large catalog (400k products) that takes a day or two to reindex.

Occasionally I’ve had something go wrong with the indexing process and then nothing is searchable (code errors). I don’t want the site down for days if this happens, so I’d like to clone the current index, then start the Magento reindexing, and if something goes haywire, restore the backup index.

I can’t find how to see/overwrite which version of the index is currently being used.