There are some keys given in the scriptures to help us, but it is also likely that we may make mistakes. Doing so is part of the process for our growth. So we need not fear that it is possible we will make mistakes as much as we are open to the idea that it is a possibility. This will keep us from settling too stridently on something and can help keep us open to correction. Part of the process is trusting that if we do make mistakes, the Lord will guide us and correct us and help us learn from them. Part of the process is trusting that the Lord will guide us back on the path as needed.
By their fruits
Moroni, in Moroni 7, taught us that when considering whether something is true, one question we can ask is what are the fruits?
When a messenger comes who says he from God and that he has brought a message from God, what are the fruits of giving heed to that message? What are the fruits of others who have given heed to that message? If you try an experiment on the word, what are the fruits within you? Are positive changes brought about? Do you become more Christ-like? Do you become a better person when you apply the message?5 For I remember the word of God which saith by their works ye shall know them; for if their works be good, then they are good also.6 For behold, God hath said a man being evil cannot do that which is good; for if he offereth a gift, or prayeth unto God, except he shall do it with real intent it profiteth him nothing.8 For behold, if a man being evil giveth a gift, he doeth it grudgingly; wherefore it is counted unto him the same as if he had retained the gift; wherefore he is counted evil before God.
A Leap of Faith
Related to the idea of fruits, is the idea of exercising faith. Many times it requires action on our part in order to be able to test the word and test its fruits. Just hearing a message is often not sufficient for knowing whether or not it is true.
This process of testing the words and exercising faith is described in Alma 32:
26 Now, as I said concerning faith—that it was not a perfect knowledge—even so it is with my words. Ye cannot know of their surety at first, unto perfection, any more than faith is a perfect knowledge.27 But behold, if ye will awake and arouse your faculties, even to an experiment upon my words, and exercise a particle of faith, yea, even if ye can no more than desire to believe, let this desire work in you, even until ye believe in a manner that ye can give place for a portion of my words.28 Now, we will compare the word unto a seed. Now, if ye give place, that a seed may be planted in your heart, behold, if it be a true seed, or a good seed, if ye do not cast it out by your unbelief, that ye will resist the Spirit of the Lord, behold, it will begin to swell within your breasts; and when you feel these swelling motions, ye will begin to say within yourselves—It must needs be that this is a good seed, or that the word is good, for it beginneth to enlarge my soul; yea, it beginneth to enlighten my understanding, yea, it beginneth to be delicious to me.
29 Now behold, would not this increase your faith? I say unto you, Yea; nevertheless it hath not grown up to a perfect knowledge.30 But behold, as the seed swelleth, and sprouteth, and beginneth to grow, then you must needs say that the seed is good; for behold it swelleth, and sprouteth, and beginneth to grow. And now, behold, will not this strengthen your faith? Yea, it will strengthen your faith: for ye will say I know that this is a good seed; for behold it sprouteth and beginneth to grow.31 And now, behold, are ye sure that this is a good seed? I say unto you, Yea; for every seed bringeth forth unto its own likeness.32 Therefore, if a seed groweth it is good, but if it groweth not, behold it is not good, therefore it is cast away.33 And now, behold, because ye have tried the experiment, and planted the seed, and it swelleth and sprouteth, and beginneth to grow, ye must needs know that the seed is good.34 And now, behold, is your knowledge perfect? Yea, your knowledge is perfect in that thing, and your faith is dormant; and this because you know, for ye know that the word hath swelled your souls, and ye also know that it hath sprouted up, that your understanding doth begin to be enlightened, and your mind doth begin to expand.35 O then, is not this real? I say unto you, Yea, because it is light; and whatsoever is light, is good, because it is discernible, therefore ye must know that it is good; and now behold, after ye have tasted this light is your knowledge perfect?36 Behold I say unto you, Nay; neither must ye lay aside your faith, for ye have only exercised your faith to plant the seed that ye might try the experiment to know if the seed was good.37 And behold, as the tree beginneth to grow, ye will say: Let us nourish it with great care, that it may get root, that it may grow up, and bring forth fruit unto us. And now behold, if ye nourish it with much care it will get root, and grow up, and bring forth fruit.
Using the Light of Christ
Every man who comes into the world is given the light of Christ, or in other words, a conscience. This gift is given that we may be able to discern good from evil. Moroni also explains that the light of Christ can be used to help us discern truth.
Does it Edify?12 Wherefore, all things which are good cometh of God; and that which is evil cometh of the devil; for the devil is an enemy unto God, and fighteth against him continually, and inviteth and enticeth to sin, and to do that which is evil continually.13 But behold, that which is of God inviteth and enticeth to do good continually; wherefore, every thing which inviteth and enticeth to do good, and to love God, and to serve him, is inspired of God.14 Wherefore, take heed, my beloved brethren, that ye do not judge that which is evil to be of God, or that which is good and of God to be of the devil.15 For behold, my brethren, 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.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.
In the early days of the LDS church, there were many who were deceived by false spirits who sought to deceive those seeking to come closer to God. We can read about this in D&C 50:
There were also many who were deceived by their fellow men who were acting as deceivers and hypocrites:1 Hearken, O ye elders of my church, and give ear to the voice of the living God; and attend to the words of wisdom which shall be given unto you, according as ye have asked and are agreed as touching the church, and the spirits which have gone abroad in the earth.
The Lord then goes on to explain another way of discerning whether a message is of God:7 Behold, verily I say unto you, there are hypocrites among you, who have deceived some, which has given the adversary power; but behold such shall be reclaimed;
17 Verily I say unto you, he that is ordained of me and sent forth to preach the word of truth by the Comforter, in the Spirit of truth, doth he preach it by the Spirit of truth or some other way?19 And again, he that receiveth the word of truth, doth he receive it by the Spirit of truth or some other way?21 Therefore, why is it that ye cannot understand and know, that he that receiveth the word by the Spirit of truth receiveth it as it is preached by the Spirit of truth?
The Holy Ghost
At the end of the Book of Mormon is a promise:
3 Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.4 And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
While this verses apply directly to coming to know if the words in the Book of Mormon are true, they show a general pattern that can be used to know the truth of all things: In particular, the truth of all things can be known by the power of the Holy Ghost.
So how do we identify the Holy Ghost? Joseph Smith taught that the Holy Ghost enlightens our understanding. The Holy Ghost delivers intelligence or sudden insight.
As we try to understand what the Holy Ghost is like, it is also important to consider these verses from the Book of Mormon in 1 Nephi 16:
1 And now it came to pass that after I, Nephi, had made an end of speaking to my brethren, behold they said unto me: Thou hast declared unto us hard things, more than we are able to bear.2 And it came to pass that I said unto them that I knew that I had spoken hard things against the wicked, according to the truth; and the righteous have I justified, and testified that they should be lifted up at the last day; wherefore, the guilty taketh the truth to be hard, for it cutteth them to the very center.
The reason it is important to keep this scripture in mind is related to the my last post on receiving truth. Sometimes, due to false traditions that we have inherited, the truth is not always easy to receive. Often, the truth requires that we repent and let go of falsehoods and unbelief. This process may not always be easy. The truth does not always re-assure us. If we have erroneous beliefs, the truth may shake us at first and require us to soften our hearts in order to receive it. It may not always make us "feel good" but it will always edify, enlighten, and increase our understanding.
Sometimes the LDS people reject that which is good because it may not make them feel good about themselves. Here is a post from another LDS blogger which I think describes this phenomenon well.
Truth will invite us to do good. Truth will invite us to increase our faith and reliance on God and bring forth good fruit. Truth is light.