Where do I view the space left on my external hard drive?

Select the hard drive in the Finder and press command-i (or choose File -> Get Info).

Edit: Alternately, you can double-click the hard drive and look at the bottom of the window. You may need to choose View -> Show Status Bar if the free space is not displayed there.

From the menu bar click on the Apple symbol and select About this Mac. Click the more info button and from the upper left hand side of the window click on storage.

If you have the status bar turned on in the Finder, you can view it there when you select the disk.

Otherwise, Get Info on the external.

Or you can do what Thomas has outlined as he is a quicker typist! 😉