How can you tell when a Prophet is speaking as a man vs for God? HD

What should if I disagree with a prophet on an issue (like gay marriage)? Can prophets be mistaken? If a prophet can make mistakes, how can we tell when what they say is true, only when we agree with them? Am I being a sheep by following someone I disagree with? Are general conference talks pure revelation? How should we deal with mistakes prophets make? Can they make mistakes? Some helpful quotes to keep in mind. ----------- The Law of Witnesses "In the mouth of two or three witnesses shall every word be established." Paul, 2 Cor 13:1 "One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established." Duet 19:15 "If I bear witness of myself, my witness is not true. There is another that beareth witness of me, and I know that the witness which he witnesseth of me is true. Ye sent unto John, and he bare witness unto the truth...But I have greater witness than that of John: for the works which the Father hath given me to finish, the same works that I do, bear witness of me, that the Father hath sent me. And the Father himself, which hath sent me, hath borne witness of me." - Jesus, John 5:31-37 "Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another? Wherefore, I speak the same words unto one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also."- 2 Nephi 29:8 “When prophets write and speak on the principles of the gospel, they should have the guidance of the Spirit. If they do, then all that they say will be in harmony with the revealed word. If they are in harmony then we know that they have not spoken presumptuously. Should a man speak or write, and what he says is in conflict with the standards which are accepted, with the revelations the Lord has given, then we may reject what he has said, no matter who he is.” (Smith, Doctrines of Salvation, 1:187.) "A few question their faith when they find a statement made by a Church leader decades ago that seems incongruent with our doctrine. There is an important principle that governs the doctrine of the Church. The doctrine is taught by all 15 members of the First Presidency and Quorum of the Twelve. It is not hidden in an obscure paragraph of one talk. True principles are taught frequently and by many." - Neil L Andersen Prophetic Infallibility? ----- When a prophet speaketh in the name of the Lord, if the thing follow not, nor come to pass, that is the thing which the Lord hath not spoken, but the prophet hath spoken it presumptuously" - Duet: 18:22 "And now, if there are faults they are the mistakes of men; wherefore, condemn not the things of God, that ye may be found spotless at the judgment-seat of Christ." - Title Page Book of Mormon. "And now, if I do err, even did they err of old; not that I woul