$ df -h ( hit enter )

Will return something similar to:

Filesystem            Size  Used Avail Use% Mounted on
/dev/sda5             128G   80G   47G  64% /
/dev/sda2             2.0G  264M  1.6G  15% /tmp
/dev/sda1              99M   23M   71M  25% /boot
tmpfs                  16G     0   16G   0% /dev/shm

To display the long listing in a human readable format that nicely summarizes the file sizes, use:

$ ls -lah slow-log
****************************
Example output
****************************
-rwxrwxrwx 1 bloom bloom 34K Dec 12 10:24 slow-log

From command line:

$ dstat -f

This will show a great report in color for your system in real time.

For more documentation:

http://dag.wieers.com/home-made/dstat/dstat.1.html

sar – Collect, report, or save system activity information.

From command line:

$ sar -b

This command shows the amount of traffic and you can check for spikes in traffic