If you are not familiar with the oc command, refer to OpenShift - Getting Started with the oc command.

The oc get clusterversion command with --output yaml or --output json can be used to determine the Cluster ID.

~]$ oc get clusterversion --output yaml
items:
  spec:
    clusterID: 36255ad8-8b26-4327-8ea0-d58c181349a4

Or, this one liner can be used.

~]$ oc get clusterversion --output jsonpath={.items[*].spec.clusterID}
36255ad8-8b26-4327-8ea0-d58c181349a4