Knowing the best course of action is not meant to be a mystery:
...it is given unto you to judge, that ye may know good from evil; and the way to judge is as plain, that ye may know with a perfect knowledge, as the daylight is from the dark night. (Moroni 7:15)
The Spirit of Christ, given to every man in the form of reason and conscience, is the key to knowing the best course of action:
16 For behold, the Spirit of Christ is given to every man, that he may know good from evil; wherefore, I show unto you the way to judge; for every thing which inviteth to do good, and to persuade to believe in Christ, is sent forth by the power and gift of Christ; wherefore ye may know with a perfect knowledge it is of God.
17 But whatsoever thing persuadeth men to do evil, and believe not in Christ, and deny him, and serve not God, then ye may know with a perfect knowledge it is of the devil; for after this manner doth the devil work, for he persuadeth no man to do good, no, not one; neither do his angels; neither do they who subject themselves unto him. (Moroni 7)
Your faculty of reason, or your ability to logically evaluate benefits and costs in the context of an understanding of reality, will make the right course of action obvious.
In plain, modern English, good and evil are questions of consequence. What is good is what benefits, what improves, what brings increased joy, and what increases resilience to suffering. What is evil is what harms, what degrades, what steals joy, and what increases susceptibility to suffering.
You can’t discern good and evil without discerning the consequence of action, and you can’t discern the consequence of action except through personal experience or the logical evaluation of the experience of others. What is good and what is evil depends upon a person’s understanding. Two important implications of this dependency are that the highest good can only be perceived by those with the greatest understanding, and that a more accurate perception of good and evil can come only through gaining understanding.