clicking on "my computer" in win 7 or "this pc" in win 10 open a window that let's you see the drives connected and then to explore each drive's files. this window is known as windows explorer as it let's you "explore" the pc.
opening it should let you see the external drive listed with a drive letter (probably e: of even f: depending on what other drives you have installed). double clicking that drive will let you see what files are on the drive and such. instead of double clicking, right click the drive and select "properties" from the menu that pops up. this will show yu the size of the drive as well as how much empty space there is on it.