MARK - How do we know when the Prophet is speaking the word of God? I thought I had read that he would say the words "Thus saith the Lord" but I can not find this.

JOEL - It has never been a requirement for the prophet to say "Thus saith the Lord", for something to qualify as doctrine, prophecy, or scripture. Many people have assumed this based on times where a prophet has spoken those words in connection with a revelation found in the scriptures, where it occurs over 230 times.
In 1980 President Ezra Taft Benson presented a list which he called "Fourteen Fundamentals in Following the Prophet". Number six in that list states the following:

"The prophet does not have to say "Thus saith the Lord" to give us scripture."

President Spencer W. Kimball said:
"Now, someone asks, "Why do the revelations come in that manner? Why don't they come preceded by 'thus saith the Lord'? " They haven't always come that way, even in the days of old. When Wilford Woodruff gave us the revelations to him concerning the discontinuance of plural marriage, he explained then that "thus saith the Lord" was used very rarely by the latter prophets, just the same as we never use anymore in our writing the words "and it came to pass." He continued:
"I made some remarks last Sunday, upon the same revelations. Read the life of Brigham Young and you can hardly find a revelation that he had wherein he said "Thus saith the Lord," but the Holy Ghost was with him; he was taught by inspiration and by revelation, but with one exception he did not give those revelations in the form that Joseph did; for they were not written or given as revelations and commandments to the church in the words and the name of the Savior. Joseph said "Thus saith the Lord" almost every day of his life in laying the foundation of this work. But those who followed him have not deemed it always necessary to say "Thus saith the Lord," yet they have led the people by the power of the Holy Ghost. I do not want the Latter-day Saints to understand that the Lord is not with us and that he is not giving revelations to us, for he is giving us revelation and will give us revelation until this scene is wound up." (Sermon of Wilford Woodruff, Logan, Utah, November 1, 1891.) (Teachings of Spencer W. Kimball, p 457)

The Lord told us in the scriptures how we should treat the words coming from the Prophet Joseph Smith:

"thou shalt give heed unto all his words and commandments which he shall give unto you as he receiveth them, walking in all holiness before me;" (D&C 21:4)

This would apply to all Church prophets today, regardless of whether they say, "thus saith the Lord" or not.

If we generally accept the current president of the church as a prophet, seer, and revelator, we can assume that whatever he says to us at General Conference; official Church manuals or magazines; official policies or statements; public speeches; or new scripture, can be considered the word of God. Any official statements or scripture coming from the prophet will always be verified and confirmed by the other Apostles.
However, it is not expected of us to blindly follow or believe everything that the prophet says. We have always been encouraged to study and seek confirmation from the Holy Ghost about anything we learn from the leaders of the church. This is the final test which can confirm to us that the prophet is speaking for God.