Many people that you meet will have something to teach you. The key question is not whether someone has a different quantity of light and truth than you—everyone will. The questions are: 1) in what area and 2) which truth is higher? It is quite easy to sense a disparity in light and truth between yourself and another. However, it is nearly impossible to tell who is the more advanced until you test the new putative truth:
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. (Alma 32)
You test a message by experimenting upon it. If you live it and improve, it is right. If you live it and do not improve, it is false. What a true messenger offers is the opportunity to experience what they claim to have experienced. If you have done what they have taught and not received what they have claimed to receive, one or both of you is a liar. Either you have not done what they have done, they have not experienced what they claim, or both. Anyone who follows a man is a fool. Anyone who follows a fool they have disproven through their lack of experiences is not only a fool, but a liar.
Anyone claiming experiences with God ought to be able to instruct others on how to obtain those same experiences. If they are unable to bring others who apply their ideas to the same experiences they claim, you can set them down as imposters. Their experiences either never happened, came from the devil, or happened long ago and they have since lost that light and replaced it with darkness through sin.
An ideal messenger of God will make himself redundant by successfully bringing up those who hear him to the point where they equal or surpass him in experiences with God, as Jethro did with Moses. Conversely, if we consider a man as the object of evaluation, instead of considering each message separately, we limit ourselves to the state obtained by that man. Unless that man is Jesus Christ, our decision will result in severe limitation on our part.