If a person claims to be a prophet of God, one of the following must be true:
1. The person knows he is a prophet, is telling the truth, and is a prophet.
2. The person thinks he's a prophet, is telling the truth of what he believes, but is deceived (by Satan, mental disorder, low intelligence, doesn't understand what a prophet is, misinterprets personal experiences, etc., etc.) and is not a prophet.
3. The person knows he is not a prophet, is intentionally lying, and is not a prophet.
In the LDS Church, 15 men claim to be prophets of God. This includes the First Presidency and the Quorum of the Twelve.
I'll start with (3). As a practicing Latter-day Saint, I know first hand of the sincerity of most members of my church. For example, I don't claim to be a prophet. But, I know in my heart that I'm sincere in my LDS convictions. Someone might argue that I'm deceived, but nobody can successfully argue that I lie about what I believe. I am no different in this regard to most other members of my church.
Nobody in my church volunteers to become leaders in the church. These opportunities come as callings from higher leaders. I could theoretically be called to be a Bishop, a Stake President, or a member of the Quorum of the Twelve. All of the 15 who now claim to be prophets were once like me, a believing church member in no particular position of prominence. If I were called to join the Twelve, would it be reasonable to think that I switched from being honest to being a liar? No, it would not be.
Based on the general honesty of rank and file Mormons and based on the fact that leaders are called from among those ranks, it is inconceivable to think that those who are called as Apostles and Prophets do not honestly believe in their calling, To be otherwise, when a person is called to high position in my church, they would have to discover that the church is not true, decide to stop believing, decide to become a liar, and then claim to be a prophet in order to propogate the falsehood. They would leave the ranks of the "deceived" to join the ranks of the "deceiver". Perhaps a couple of people here and there could change stripes from integrity to liar over the years, but not the entire group of those who are called to be Prophets and Apostles.
I, therefore, find (3) to be an impossible and an illogical idea, based on my firsthand experience in the LDS Church.
Let's discuss (2). All 15 men claim to be prophets and apostles. They claim to have had sacred spiritual experiences, after which God specifically commanded them to not reveal it, due to its sacredness, and due to the fact that many would ridicule. Joseph Smith was commanded to speak freely of his revelations, but in our day, the prophets have been commanded to not speak so freely of theirs. Nevertheless, all 15 of these men make it clear that they have each had such revelatory experiences. They testify that they have absoulte, positive knowledge of Jesus Christ, by revelation. If you're Christian, think of the certitude of Moses, Noah, Peter, or John as to what they believed. Their revelatory experiences lifted them beyond just believing into having a factual knowledge, as certain as they know that they exist. These 15 men in the LDS church claim to have the same revelatory knowledge.
Since we're on (2), let's consider if 15 men can be so deceived? It would not just be 15, but all 15 over the years from Joseph Smith forward. These men are highly intelligent. I already established their honesty. They come from the ranks of educators, heart surgeons, state supreme court judges, Harvard professors, scientists. and other reputable positions. They are men of great accomplishment and talent. These are not men who decided at some point in life that they wanted to enter the ministry because they felt "called". These are men who were literally plucked up from what they were doing and asked to serve in their current church position.
Knowing what I know about these 15 men, it's inconceivable to me that they are all deceived. They are too well grounded to all believe that they have had these miraculous revelations when they really didn't happen. It would be like believing that all 11 Apostles were deceived when Christ appeared to them after the resurrection and told them to touch his body and see for themselves.
If 2 and 3 are inconceivable, that leaves 1. Think about it.