A most sensitive area of constructive speech is that of kashrus.
Raising doubts regarding the reliability of a hechsher (rabbinical endorsement) without reliable information constitutes speaking loshon hora. If one has reason to suspect that a hechsher is inferior, the matter should be discussed with a competent rav, who can offer guidance as to how one should proceed. Even if one rav suggests that a hechsher not be relied upon, this does not necessarily mean that the hechsher is definitely unreliable. It is the rav of the community who should decide the status of a hechsher for his constituents, or else direct them to a recognized posek (halachic authority) for his decision.
In a case of a questionable product manufactured by a local Orthodox Jew who assumes responsibility for his product and does not operate under a hechsher, extreme caution must be exercised before declaring the product non-kosher. Only the local rav or posek is in a position to decide the halachic status of the item, after taking all factors into consideration. Causing a Jew to close down his business is a very serious matter and preventing such a situation may even outweigh the observance of chumros (unrequired halachic stringencies) generally kept by the community.
Sefer Chofetz Chaim, 16 Kislev, Page 200