Why Ask?
The Watchtower, 4/1/72, p. 197 : “However, Jehovah did not let the people of Christendom, as led by the clergy, go without being warned that the League was a counterfeit substitute for the real kingdom of God. He had a “prophet” to warn them. This “prophet” was not one man, but was a body of men and women. It was the small group of footstep followers of Jesus Christ, known at that time as International Bible Students. Today they are known as Jehovah’s Christian witnesses. They are still proclaiming a warning, and have been joined and assisted in their commissioned work by hundreds of thousands of persons who have listened to their message with belief. Of course, it is easy to say that this group acts as a “prophet” of God. It is another thing to prove it. The only way that this can be done is to review the record. What does it show?”.
Bible: 1John 4:1 NASB - “Beloved, do not believe every spirit, but test the spirits to see whether they are from God because many false prophets have gone into the world”.
How Determine?
Deut. 18:22 NASB - “When a prophet speaks in the name of the LORD, if the thing does not come about or come true , that is the thing which the LORD has not spoken. The prophet has spoken it presumptuously ; you shall not be afraid of him”.
If one prophecy fails, the person(s) are NOT a prophet.