Before relating negative information for constructive purpose, one must carefully consider whether or not his words will, in fact, bring about the desired result. Speaking negatively for constructive purpose is justified only if there is a reasonable chance that the desired result will be accomplished.
(Taken from the Sefer Shmiras Halashon, Chofetz Chaim A Lesson A Day 14 Chesvhvan, Day 43 pg. 136)