You are forbidden to say that a person did something improper, even if you are motivated by kinaas ha’emes – that is, you are agitated about the transgression and insistent that the truth be known. Although your words might be meant l’shaim shomayim and are without malice, they still constitute loshon hora. You are allowed to relate details of an improper action only if your intention is constructive. ~ Guard Your Tongue
For example, you cannot express your annoyance to someone that someone else is not careful with kashrus. However, if his sole intention is to inform him of this so that he doesn’t eat at this person’s house, that would be permitted.