Enabling the Mac Keyboard Viewer
OS X's Keyboard viewer is a useful way to find what characters may be typed (within the current input language). The layout defaults to the current keyboard. Note that the current input language is listed at the top of the viewer.
Viewer reflects the current keyboard
If an external keyboard is used, the display will update to reflect the keyboard currently sending keystrokes.
Pressed key(s) are highlighted
A pressed key is highlighted in grey. Here, the colon key is being pressed. Pressing Opt+k would highlight both the Option key and the k key. The key-presses are undated in real time - you can watch yourself typing and see all the key-presses.
The effect of modifier keys -
Try holding down in succession, Option and then Shift+Option. See how the Viewer updates to show a different character that a key will type with such a modifier. Notice how when just the Option key is held down (grey highlight above) some keys are shown in orange. This indicates keys used for typing accented characters. Typing Cmd+u and then 'a' gives ä. In this way an umlaut, circumflex, acute, grave and tilde accents can be applied to any Roman character that supports that accent pairing (the current font's support for that glyph can also be a limiting issue - i.e. you can type it but the font doesn't have a character defined for it).
Mouse-clicking a key on the Viewer palette also inputs the currently displayed character for that key. For some people this may be an easier way to type in less usual characters.
With a British keyboard-input, to type a German ß, Opt+S correctly gives ß. But if typing a long passage in German, it may help to add German as an Input Language (explained further below) and switch to it. Here you see Faroese loaded, enabling an Eth character to by typed directly, like so: ð. Of course, the author's physical keyboard has not changed, so the key used to type the ð is physically marked as ] on the UK keyboard. Indeed, with Faroese input still set it requires typing Opt+9 to create the ], which is a bit confusing!
However, this does offer a lot of flexibility for input. Changing the input languages does not affect characters already typed, just those typed subsequent to the change of input.
Note that changing the input language simply affects refers to what you type (into the frontmost app). It is entirely separate from using a different language for the OS, which would change the language of all the OS menus, etc. If you need the latter ,you want the Language tab of the Language & Text preferences.
A change in input can go beyond making it easy to add the odd extra accented character in Roman alphabets. Input in Cyrillic or Arabic and the like are possible (assuming you understand those languages). Above, the viewer is showing Arabic characters for Afghan Pashto. In such a context, it is likely you will want to leave the Keyboard Viewer open as your physical keyboard key captions will look very different from the characters you will be typing. Also, if using an unfamiliar language (perhaps to add a quote), don't assume an English 'a' key simply types the 'a' equivalent in another language.
Setting up the Keyboard Viewer & multiple languages
Open System Preferences and then Language & Text. In older OSs (pre Snow Leopard) the latter may be titled differently.
In the Language & Text preferences, select the Input Sources tab.
Enable the Keyboard Viewer as an input
At the top of the left-side list of input methods (languages), ensure the top item is ticked: Keyboard & Character Viewer.
Not covered in this tutorial, the Character Viewer allows you to find and view any Unicode character. Perhaps you want to find things like the Mac key symbols like â and â¥ (for the Command and Option/Alt keys).
Enable Viewer access via the OS X menu bar
Tick the box bottom right of the preference pane. A new icon appears - note it is inserted to the right of any third-party apps' menu icons.
The Viewer Menu (only one input language set)
With only one input language set - the default for most users, the men icon and menu is as above. Note the icon used is that of the Character viewer. The menu items have a show/hide option depending on whether the Viewer is already open. Here the character Viewer is closed and the Keyboard Viewer is open. The "Open Keyboard…" option opens the Keyboard section of System Preferences.
The Viewer Menu (multiple input languages set)
With multiple input languages set, here British/Faroese/German, a flag & name for each current input language is added. The currently selected input language is ticked in the menu and its flag used as the menu bar icon to remind you of the input language in use as it may differ from that for which your physical keyboard(s) are marked. Thus changing input languages will change the flag seen in the menu bar. At the same time, the Keyboard Viewer will update to show the actual characters keys will type in the selected language.
Note too, some extra menu options. The "Show Input Source Name" will show the language name (as per the menu list) to the right of the icon in the menu bar. The "Open Language & Text…" option opens said preferences and helpfully pre-selects the Input Sources tab.
Selecting/de-selecting input languages
To remove a current input language from the list simply un-tick it. When opening these preferences, all selected items are listed first then other available options are listed in alphabetical order. To refresh the list ing to show newly added choices at the top of the list or lose de-selected ones you must completely close and re-open System Preferences; just changing the current tab or another preference pane won't refresh the list.
Remember the menu bar icon only shows a language flag if more than one input language is selected.