(define (sorted? lon)
  (or (null? lon)
      (null? (cdr lon))
      (and (<= (car lon) (cadr lon))
	   (sorted? (cdr lon)))))

; A shorter way:
(define (sorted? lon)
  (or (null? lon)
      (apply <= lon)))

(sorted? '())
(sorted? '(3))
(sorted? '(2 4 5 3 6))
(sorted? '(2 3 4 5 6))