For a statement to be excluded from the realm of rechilus and be classified instead as constructive, several conditions are necessary.
Most important is that one ascertain that the information is fully accurate. This involves both verification of the facts as well as a clear understanding of the situation (as was discussed regarding conveying negative information for a constructive purpose).
The commandment, “Do not stand aside while your fellow’s blood is being shed” (Vayikra 19:16), obligates us to report constructive information if our knowledge of it is firsthand. Should one choose to speak of an alleged occurrence that he did not personally witness in order to save his fellow Jew from possible harm, it must be clearly stated that the information is based on hearsay and is not to be accepted as fact.