How to identify the storage space left in a persistent volume claim?
If there's a running pod with mounted PV from the PVC,
kubectl -n <namespace> exec <pod-name> df
...will list all file systems, including the mounted volumes, and their free disk space.
You can monitorize them with kubelet prometheus metrics:
kubelet_volume_stats_available_bytes{persistentvolumeclaim="your-pvc"}
kubelet_volume_stats_capacity_bytes{persistentvolumeclaim="your-pvc"}