This assumes:

• You have an ArgoCD server up and running. If not, check out my article Install Red Hat OpenShift GitOps Operator using the console (Argo CD)
• You have installed the ArgoCD CLI.
• You are able to log into ArgoCD using the CLI. If not, check out my article Log into ArgoCD using the CLI on OpenShift

The argocd admin cluster namespaces command can be used to list the namespaces in each cluster that ArgoCD manages.

~]$ argocd admin cluster namespaces
CLUSTER                                                      NAMESPACES
https://kubernetes.default.svc
https://api.prod.op.example.com:6443                         foo,datadog,bar,hello-world (total 11)
https://api.dev.op.example.com:6443                          foo,datadog,bar,hello-world (total 13)
https://api.stage.op.example.com:6443                        foo,datadog,bar,hello-world (total 16)