How can you discern whether or not the words you are about to speak are called by God?
Are they true? According to what you have been taught by Jesus, and according to God's Law, are the words reflective of God's enduring will for justice and peace and abundant life for each person?
Are they loving? This doesn't necessary mean that they will be seen as "gentle" or "affectionate". It means: Are the words reflective of the love that God has for each of us? Even if they are tough words that the listener may not want to hear, can they be spoken with an intention rooted in love of God, for all people, and for all of creation?
Are they necessary? As you pray about whether or not to speak, does it seem that the words on your tongue must be spoken so that the situation hears a directive that can move it closer to God's Will?
If the answers to the three questions are all "yes" you can ask for God's help in speaking them. Do your best, and wherever you fall short, God will most certainly forgive you and "pick up the slack"!