(defgeneric foo (bar))
(defclass fred () ())
(defmethod foo ((bar (eql (find-class 'fred)))))
(setf (find-class 'fred) nil)
(defclass frob () ())
(defmethod foo ((bar (eql (find-class 'frob)))))
(setf (find-class 'frob) nil)

(generic-function-methods #'foo)
#|
(#<STANDARD-METHOD PROTEST/PROTOCOL::FOO ((EQL #<STANDARD-CLASS PROTEST/PROTOCOL::FROB {1002E262B3}>)) {1002E66D03}>
 #<STANDARD-METHOD PROTEST/PROTOCOL::FOO ((EQL #<STANDARD-CLASS PROTEST/PROTOCOL::FRED {101CEC1203}>)) {1002BA5DF3}>)
|#