OK. Via CLI, you go to NFS Directory ( same GlusterDriver , just go to corresponding folder of Gluster Volume brick) , where cinder volumes reside.
Total space available
$ df -h .
Space have been used
$ du -m -s .

 Command `cinder list`  && `cinder show volume-id` will show you space allocated for every volume been created
   as list and detailed for particular volume.

The following command (got tip from a colleague) worked for me to get the used and total GBs of my volume storage:
openstack limits show --absolute --sort-column Name | grep Gigabytes
Hope it helps for the googlers finding this page.

I know this question is old but doesn't seem there's a straight answer for it. Here's how i find out how much available space is left on my cinder-volume.

The command is "sudo vgdisplay cinder-volumes" .. cinder-volumes is name of my cinder volume.

user1@Compute1:~$ sudo vgdisplay cinder-volumes --- Volume group --- VG Name cinder-volumes System ID Format lvm2 Metadata Areas 1 Metadata Sequence No 22 VG Access read/write VG Status resizable MAX LV 0 Cur LV 5 Open LV 5 Max PV 0 Cur PV 1 Act PV 1 VG Size 3.64 TiB  <-- ACTUAL SIZE PE Size 4.00 MiB Total PE 953861 Alloc PE / Size 28416 / 111.00 GiB <-- SPACE ALLOCATED Free PE / Size 925445 / 3.53 TiB <-- SPACE AVAILABLE VG UUID bMX896-RO4S-4PuK-iJ11-vxWY-oFtp-pGdXTG

user1@Compute1:~$