R4RS 6.9  (procedure? object)  ==>  boolean

Returns #T if OBJECT is a procedure, otherwise returns #F.

(procedure? car)                    ==>  #t
(procedure? 'car)                   ==>  #f
(procedure? (lambda (x) (* x x)))   ==>  #t
(procedure? '(lambda (x) (* x x)))  ==>  #f
