In the Command Line type in the following command: df -h and press enter.

Results will look similar to the below.

Filesystem            Size  Used Avail Use% Mounted on /dev/sda7              74G   36G   35G  51% /

Explanation of command:

df = report file system disk space usage -h = print sizes in human readable format (e.g., 1K 234M 2G)