The command M-x eieio-browse displays a buffer listing all the
currently loaded classes in Emacs. The classes are listed in an
indented tree structure, starting from eieio-default-superclass
(see Default Superclass).
With a prefix argument, this command prompts for a class name; it then lists only that class and its subclasses.
Here is a sample tree from our current example:
eieio-default-superclass
+--data-object
+--data-object-symbol
Note: new classes are consed into the inheritance lists, so the tree comes out upside-down.