You can use Key Caps from the Apple menu. Key Caps shows you the keys settings when holding down the shift, option, or shift and option together. For example if you hold down the Option key, you can see how the mapping changes.
The umlaut is a "dead key" and is properly mapped by the Mac OS. That is, when you press the key combination to get an umlaut, you don't see the character until you press the letter that the umlaut is to appear over. For example, the keystrokes Option-u, followed by "u" will produce an umlaut over the letter "u".
Make sure you know where the Option and Command keys are mapped in MAE. For convenience, you may want too map them to the same physical location on the workstation keyboard as they are on a Macintosh keyboard, but this is not necessary. You can use the MAE Keyboard control panel to check their location and remap them if desired.
Several third-party Macintosh developers provide Apple menu item and control panel device utilities that also provide this functionality.
You can also try to map the same key combination you normally use in X Windows, for example map the dead key or compose key that X Windows use in MAE. Search for Tech Info Library article titled "MAE: Keypad Mapping On Solaris & Some Mac Applications" for more information
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