CONSP function returns true if the argument refers to cons cell, otherwise it returns false. See CONS and LIST.

(consp nil) => NIL
(consp 'some-symbol) => NIL
(consp 3) => NIL
(consp "moo") => NIL
(consp (cons 1 2)) => T
(consp '(1 . 2)) => T
(consp '(1 2 3 4)) => T
(consp (list 1 2 3 4)) => T