A good administration practise is the check the disk space usage from time to time to prevent disk full. The smooth running of a system requires storing system logs, writing temporary files, spooling emails and print jobs, etc. When disk spaces are unavailable, these operations would fail.
To check the usage of "mounted" disk space, we can use command "df".
e.g.
[yuni@lovely-linux yuni]$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda1 782664 621976 120932 84% / /dev/hda2 2229548 1972568 143724 94% /usr /dev/hda4 2411408 2077756 211160 91% /home
N.B. your output may be different.
In this example, the root filesystem on /dev/hda1 has used up 608MB out of its total size 764MB. The usage of other two filesystems /usr and /home are 94% and 91% respectively.