Greek characters can be created using the Symbol font. For other characters, go to System Preferences > Language & Text > Input Sources and enable Keyboard & Character Viewer. Also make sure Show input menu in menu bar (at the bottom) is enabled. Click on the keyboard or flag icon in the menu bar. Select Show Keyboard Viewer. You now have access to all of your active fonts and what they look like according to your keyboard layout. Everything changes dynamically as you hold various keys such as shift, command, option, etc.
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