Sefer Chofetz Chaim Hilchos Rechilus chapter 9 halacha 2

There are 5 conditions that must be met in order to be allowed to speak rechilus for a constructive purpose. 1. Can’t jump to conclusions. When you see or hear something wrong that might effect others first think about what just happened, maybe even investigate before concluding it really is bad.  2. Don’t exaggerate, if you must say something just say it how it is and nothing more.  3a. One must have proper intent to help the would be victims, not to speak up out of hatred because then it would be rechilus. However that does not exempt you from speaking up because there is a mitzvah to not just stand by and let other people get hurt physically and monetarily, לא תעמוד על דם רעך. So if you have hatred towards the perpetrator this should arouse you to remove it from your heart before speaking up but if you don’t you still must speak out and you will just get a sin of rechilus for performing the mitzvah of saving a person’s life.  3b. Even if you have the proper intentions, you aren’t exaggerating, and you got the scenario right, but if there is no way that what what you say will help in any way then you can’t say it and it is rechilus. Whether you think you won’t be listened to or whether after the fact you say I told you so, after warning him once it is still rechilus because nothing good can come out of what you say. Another example is in shidduchim. You are able to speak up before a shidduch goes through, while they are still investigating, but once there is an engagement it is rechilus to speak up now, they are planning on getting married, they want each other, it is only hurtful to say anything about either party now. It is better for them to get married, and you’ll see what will happen. Maybe what you know isn’t a factor, no red flags came up during dating so your issue might not be an issue. Worst comes to worst they will have to get divorced. Another example is if the victim that you want to help has a big mouth, and he will tell the world what you said and it might even get back to the person you were talking about, there is than mo mitzvah to tell him because it might create a big fight. It’s his problem now if he might get hurt.    4. If you are able to resolve the issue without needing to speak rechilus you have to fix the situation in that manner, even if that will be the hard way of taking care of things.   5. Even if you know there is a problem, there is no other way to fix it besides speaking up, you have the right intent, to help, and you are ready ti say precisely what you heard or saw, and you know you might very likely help the potential victim but if he might cause more harm to the would be perpetrator than what any court would do to him then you can’t tell the potential victim because he can’t be allowed to punish the perpetrator more than what he deserves. That means, for example if you are the only person telling him, he can’t do anything worse than forcing the perpetrator to swear that he did, or will do no harm, because at best a single witness can only enforce an oath in court. But even if two people let say tells the victim Who stole from him. If they know the victim will go and beat up the thief, they can’t tell the victim because the court would only monetarily punish him and the victim would be going overboard, so it would be better just to tell police or the court and let them handle it for the victims instead of the perpetrator getting unlawfully punished.