This doesn't really seem a deep elasticsearch question.
Please be aware that elastiflow is not from elastic, so if there are any product dependent questions, please reach out to the author here:
That said by looking to the Kibana documentation site:
It shows that the default index seems to be index elastiflow-flow-ecs-* in elasticsearch.
So you could use the cat indices API and check if there are indices in elasticsearch:
GET /_cat/indices/elastiflow-flow-ecs-*
and check with search if there are data in the particular index:
GET /elastiflow-flow-ecs-*/_search
For a deeper inspect, kibana discover might be better.