All the return objects from API calls in NEST implement:

public interface IResponse
{
    bool IsValid { get; }
    IElasticsearchResponse ConnectionStatus { get; }
    ElasticInferrer Infer { get; }
}

IsValid will return whether a response is valid or not. A response is usually only valid when an HTTP return result in the 200 range was returned. Some calls allow for 404 to be a valid response too, however.

If a response returns 200 in Elasticsearch, sometimes it will contain more information on the validity of the call inside its response. It’s highly recommended to read the Elasticsearch documentation for a call and check for these properties on the response as well.

ConnectionStatus is the response as it was returned by Elasticsearch.Net. Its section on handling responses applies here as well.