A friend of mine who was recently baptized asked, when the modern prophets today speak, how do we know what is scripture and what is not? Sometimes my friend has a difficult time relating to doctrine that is not backed up by scriptures. He believes in the modern prophets, but wants to know how to distinguish material that could be "canonized" and material that couldn't. Do you have any thoughts on this?
I'll let the Lord himself answer your question:
"And, behold, and lo, this is an ensample unto all those who were ordained unto this priesthood, whose mission is appointed unto them to go forth-And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost. And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation. Behold, this is the promise of the Lord unto you, O ye my servants." (D&C 68:2-5)
A logical follow-up question might be: "How can one know when the brethren are being moved upon by the Spirit?" And the answer would be:
"... [He] that receiveth the word by the Spirit of truth receiveth it as it is preached by the Spirit of truth ... Wherefore, he that preacheth and he that receiveth, understand one another, and both are edified and rejoice together." (D&C 50:21-22)
Or as President J. Reuben Clark once declared:
"We can tell when the speakers are 'moved upon by the Holy Ghost' only when we, ourselves are 'moved upon by the Holy Ghost.' In a way this completely shifts the responsibility from them to us to determine when they so speak." (J. Reuben Clark Jr., "When Are the Writings and Sermons of Church Leaders Entitled to the Claim of Being Scripture?", Address to Religious Educators, 7 July 1954, at BYU)
One more thing: Your friend doesn't need to worry about "doctrine that is not backed up by scriptures." Take a look at these words from President Harold B. Lee:
"If there is any teacher who teaches a doctrine that can't be substantiated from the standard church works—and I make one qualification, and that is unless that one be the President of the Church, who alone has the right to declare new doctrine—then you may know by that same token that such a teacher is but expressing his own opinion.
If, on the other hand, you have someone teaching a doctrine that cannot be substantiated by the scriptures, and more than that, if it contradicts what is in the standard Church works, you may know that that person is teaching false doctrine, no matter what his position in this church may be.
The President of the Church alone may declare the mind and will of God to His people. No officer nor any other church in the world has this high and lofty prerogative. When the President proclaims any such new doctrine, he will declare it to be a revelation from the Lord." (Stand Ye In Holy Places, pp.109-110)