On the Load Balancer details page of the GCP Console, click on the Monitoring tab, select your backend service, and look at the graphs.

In Stackdriver Error Reporting, look for any unacknowledged errors for the Cloud Load Balancers service.

In Stackdriver Monitoring, select Resources > Metrics Explorer and search for https/request_bytes_count metric.

In Stackdriver Monitoring, select Resources > Google Cloud Load Balancers and review the Key Metrics graphs in the dashboard.

In Stackdriver Monitoring, create a new dashboard and track the https/backend_request_count metric for the load balancer.