Disk space checking

The easiest way to check your system's disk space is by using the df (disk free) command in a terminal window.

The df utility shall write the amount of available space and file slots for file systems on which the invoking user has appropriate read access. File systems shall be specified by the file operands; when none are specified, information shall be written for all file systems.

Syntax: df — report free disk space.

Options: The following arguments shall be supported:

      -a        include pseudo, duplicate, inaccessible file systems
      
      -h        print sizes in power of 1024 (e.g., 1023M)
      
      -k        Use 1024-byte units, instead of the default 512-byte  units,  when qwriting space figures.

      -P        Produce output in the format described in the STDOUT section.

      -T        print filesystem typedf -l
      
      -i        list inode information instead of block usage
      

Examples of the df command with each argument used

df

df -a

df -h

df-k

df-P

df-T

df-i

What is Filelight

For those that perfer a graphical disk usage program.

Filelight allows you to quickly understand exactly where your diskspace is being used by graphically representing your file system as a set of concentric segmented-rings. You can use it to locate hotspots of disk usage and then manipulate those areas using Dolphin or Konqueror.
You can find it in the menu -> File Tools -> Filelight.
If you can not locate it there, you may need to be installed through synaptic package manager.

Here is an example of my hard drive looking at my Nextcloud folder: