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)