Three things are necessary for the individual to begin discernment. Discernment is really understanding what God wants you to do. The three things that in my opinion are necessary are:
1. belief in God ( recognize that their is a Creator, an all knowing, omnipotent Being , who always was, always is and always will be.)
2. you are not God ( you are His creation and in that you have meaning and a purpose)
3. Heaven is the result of following God's will, Hell is the result of not following God's will.
Those three beliefs are the foundation of beginning to walk the path of meeting up with God. There are a number of means towards the practice of discernment. I favor the Spiritual Exercises of St. Ignatius. I would say the the overwhelming ingredient necessary is time in solitude and prayer. Without the time given to understanding God and getting to know Him and His will for you, it is virtually impossible to grow spiritually. Prayer is the process by which we communicate with God. The Catholic Church states: " Prayer is a vital necessity. Proof from the contrary is no less convincing: if we do not allow the Spirit to lead us, we fall back into the slavery of sin. How can the Holy Spirit be our life if our heart is far from him? Nothing is equal to prayer; for what is impossible it makes possible, what is difficult, easy.... For it is impossible, utterly impossible, for the man who prays eagerly and invokes God ceaselessly ever to sin. Those who pray are certainly saved; those who do not pray are certainly damned” CCC2744 Finally the three most important items in testing the truth versus error is to: 1. Examine everything 2. Cling to what is good 3. Avoid evil. Of course we are incapable of doing these things without God's grace.