I think it's very broad a question, How can we know if our input helps? the answer is, we don't know. But if you flip it, the same is true, how do we know our input doesn't help anyone? The answer is the same, we don't know. At which point I believe what we do, specially when giving input to someone else, or feedback, is to be as sincere as possible in our intent.
And that requires some work, and honesty. Maybe we're giving input because WE want to see that person be well, for self serving reasons, but even if that is true, it doesn't mean it won't help that person, and ourselves as we learn about us and our inclinations. In that case, I think what matters is not only intent, but how sincere and honest we are with ourselves about our intentions and their origin.