Glenn Fleishman on the Mac keyboard most of us never use. Keyboard Viewer:
Holding down Option and Shift-Option would reveal the secrets of π and ‰ and accent marks… you can click keys and those characters are inserted wherever your cursor is.
Cool. Where is Keyboard Viewer? System Preferences > Keyboard > Input Sources:
In the Keyboard preference pane’s Input Sources tab, you can click the + button in the lower left and add keyboards of many languages. Check Show Input Menu in Menu Bar, and a tiny keyboard/Command key icon appears in the upper-right corner of your screen. (If you have multiple keyboards added, a flag or icon appears that’s associated with the keyboard currently selected, instead.)
Click Show Input Menu In Menu Bar to activate.