We can know that if we pray, some outcomes will be different than if we do not pray because God made many promises conditioned on whether we ask. These promises relate to a galaxy of concerns and needs that are important to us and God, such as faithfully serving God and experiencing his blessings in our life, health, marriage, children, family, and job. If we fail to pray about everything as the Scripture commands (Phil 4:6), we will enter heaven and learn there were many things God would have done in and through us if only we'd asked!
Sadly, Calvinism's determinism has turned these wonderful promises into nothing more than a promise that God will do what he predetermined to do regardless of whether we pray or not. Do not allow Calvinism's deterministic beliefs and distortions of Scripture to rob you of this blessed intimate prayer relationship with God.