How can I check the remaining disk quota?

Take a look at the quota command here:

http://linux.die.net/man/1/quota

quota

For example:

quota -u user1

System response:

Disk quotas for user user1 (uid 501):
     Filesystem  blocks   quota   limit   grace files   quota   limit  grace    
     /dev/hda6     992   50000   55000              71   10000   11000

quota report

Report on all users over quota limits:

quota -q

Quota summary report:

repquota -a

Report for user quotas on device /dev/hda5
Block grace time: 7days; Inode grace time: 7days
                    Block limits                File limits
User            used    soft    hard  grace    used  soft  hard  grace
----------------------------------------------------------------------
root      -- 4335200       0       0         181502     0     0
bin       --   15644       0       0            101     0     0
...
user1     --    1944       0       0            120     0     0    

No limits shown with this user as limits are set to 0.

If quotas are set, "quota" command shows the details.

quota -v

try this command

repquota -sa | grep -v "0       0"

This will give details report for all user-driven accounts like:

/home]# repquota -sa | grep -v "0       0"
*** Report for user quotas on device /dev/root
Block grace time: 7days; Inode grace time: 7days
                        Block limits                File limits
User            used    soft    hard  grace    used  soft  hard  grace
----------------------------------------------------------------------
usera      --  20018M    977G    977G           351k     0     0
userb      --  11301M  55000M  55000M           106k     0     0
users      --   35192   1000M   1000M           2092     0     0