Here’s proof that you can demonstrate for yourself.
If a man’s words ring true to you, speak to your soul, edify you, call forth your own intuition, or induce the spirit of revelation in you, then you can safely assume that, at that particular moment in time, the man is in touch with God and is speaking truth.
In that brief moment in time, he is a prophet, as are we all, given similar circumstances. But, to automatically and unconditionally proclaim a man to be a prophet in every circumstance, and because of that, to follow him unconditionally, is idolatry of the worst kind.
There are certain passages in the D&C and certain speeches like Joseph Smith’s address to the Twelve, Oliver Cowdery’s address to the twelve, and Joseph Smith’s final two addresses that stir me to the very depths and make my bones to quake. There are other occasions when he was a blustering fool. He created drama and crisis wherever he went, and seemed to thrive on it, by stirring up jealousy, suspicion, and keeping everybody else off balance, to his advantage.