The du command is a useful tool to check the size of directories and files on a filesystem or inside a directory. However, its output is not the most intuitive, especially if you want to probe further into child directories trying to find what is occupying space. There are ncurses console tools like ncdu and GUI tools like Baobab that help with this. Another good alternative I have discovered is xdiskusage. xdiskusage is a GUI application that parses the output of du to show the disk space occupied by directories and files. Written in the FLTK toolkit, it is super-light and is possibly the lightest and fastest disk usage tool I have used. That makes it great for using over SSH sessions too. It uses the treemap visualization, so it is much easier to investigate disk usage than looking at du output.
$ sudo apt install xdiskusage
Invoke without any arguments if you want to be presented with a list of all the filesystems on your computer. Double-click on any to view treemap visualization of its contents.
To look under a particular directory, pass it as the first argument to the program.
In the treemap visualization, you can keep probing further by double-clicking on any rectangle.
Right-click in the GUI to get menu of all possible actions using mouse. You can also see the keyboard bindings available for browsing using keyboard.
Tried with: xdiskusage 1.48 and Ubuntu 16.04