Querying SETF expansions

If I have defined a SETF /function/ like

  (defun (setf foo) (new-value ...) ...),

then I can check for this function with

  (fboundp '(setf foo)).

However, if I have instead used DEFINE-SETF-EXPANDER or DEFSETF, is
there a way - portable or not - to find out if such a definition
exists?  FBOUNDP returns NIL in this case, MACRO-FUNCTION doesn't
accept general function names as arguments, only symbols.  Anything
else that I'm missing?  Anything LW-specific maybe?

Thanks,
Edi.