According to googles documentation, when you create a new container cluster using GUI or gcloud, Slackdriver is automatically configured with a sink, however It did not work out of the box for me hence this post :)
Well, the easiest way to check if Slackdriver is configured is to issue a command
gcloud container clusters describe If the response contains something like monitoringService: monitoring.googleapis.com then yer all good and you can simply go and visit https://app.google.stackdriver.com/gke link which should point you to you cluster.
other wise issue this command to register sinks with monitoring.googleapis.com
gcloud alpha container clusters update –monitoring-service=monitoring.googleapis.com
Hope this helps