Protection from the Truth

I have noticed that we sometimes keep things from people which we know to be true. We do this to protect them at times. We imagine that if it were us, we would not like to hear it. How do we decide whether to tell someone the truth? I remember three criteria which I heard once and which applies to anything we might say. Ask yourself, "Is it true, is it kind and is it purposeful." If what you have to say does not meet all three criteria, keep it to yourself. The hardest part for me is the third. How do I know whether what I have to say serves a purpose for my audience. Sometimes I have to listen first to get a sense of how something might be received. I can go from past experience of how a person reacts. Sometimes I just have to guess.