Answer:

Checking disk usage on Ubuntu can be done by running the command below.

df -h

The output of this command will show used, available and percentage of disk space used.

For example: 

Filesystem       Size  Used Avail Use% Mounted on
/vz/private/245   60G   37G   24G  62% /
none             2.0G  4.0K  2.0G   1% /dev
none             4.0K     0  4.0K   0% /sys/fs/cgroup
none             410M  1.1M  409M   1% /run
none             5.0M     0  5.0M   0% /run/lock
none             2.0G   24K  2.0G   1% /run/shm
none             100M     0  100M   0% /run/user

The -h flag will make the output of df command human readable.