Sorry for the confusion - my suggestion was applicable to a MULTILEADER entity, not a LEADER.
For a LEADER entity, you can either use the ActiveX arrowheadblock property as @ronjonp correctly suggests, or query the arrowhead block associated with the Dimension Style or Dimension Style override used by the LEADER entity as demonstrated by the following function:
(defun leaderarrowheadblock ( ent / def enx xdd )
(if
(and
(setq enx (entget ent '("acad")))
(setq xdd (member '(1070 . 341) (cdadr (assoc -3 enx))))
(setq def (handent (cdr (assoc 1005 xdd))))
)
(cdr (assoc 2 (entget def)))
(cdr (assoc 5 (tblsearch "dimstyle" (cdr (assoc 3 enx)))))
)
)