Found a way to see fields stored in the index:

$ curl -H 'Content-Type: application/json' -X GET pc-c2e11-35-01.cms:9200/cdaq-fmminputdetail_2018-09-21t07:28:00z_p1dt4h13m/_search?stored_fields=*

Values created by using "copy_to" can only be stored in the index and cannot be stored in the original document, i.e., in _store.