After scheduling the nightly-snapshots policy to run or nightly-snapshots run as per schedule, you can retrieve the policy to get success or failure information. You can ensure periodically that the snapshots are successfully executed as per schedule using command: curl -X GET http://{elastic_ip}:9200/_slm/policy/nightly-snapshots?human&pretty.
NOTE:The elastic_ip can be either the IP of the node in case you are in dashboard cluster or 127.0.0.1, in case you are not using cluster configuration.