The function structurep
returns true if object is of type structure-object
and false otherwise.
(structurep #(1 2 3)) => nil
(defstruct my-struct a)
(structurep (make-my-struct)) => t
but metaclasses are not structures so:
(structurep (find-class 'my-struct)) => nil
LispWorks User Guide and Reference Manual - 13 Feb 2015