How to check the used disk space of Google Persistent Disk without looking into each of them?
You can't see it in the disk preview as Faizan said, But you could Use Google StackDriver Monitoring tool to get a nice preview of your instances status.
The workaround above with snapshot size will work in the console.
For more precise (and quicker) detailed overview you can SSH into your instance and do:
sudo df -h
The response you get will look similar to this:
YOURUSERNAME@YOURINSTANCENAME:~$ sudo df -h Filesystem Size Used Avail Use% Mounted on /dev/sda1 9.8G 9.3G 0 100% / udev 10M 0 10M 0% /dev tmpfs 743M 81M 663M 11% /run tmpfs 1.9G 0 1.9G 0% /dev/shm tmpfs 5.0M 0 5.0M 0% /run/lock tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup /dev/sdb 197G 60M 197G 1% /mnt/new_disk
Entries starting with /dev/ under 'Filesystem' are your disks.
I use a simple workaround for this.
Create a snapshot of your persistent disk, and look at the snapshot size: