One of the key tests of any prophet (or prophetess) is whether or not their prophecy aligns with the revealed word and will of God. A prophet, for example, recently predicted the second coming of Jesus Christ on a particular day in this past decade. The day came and went without the predicted physical return of Jesus Christ. The man, who is now dead, was a false prophet.
Another example would be the former Worldwide Church of God (aka Armstrongites after the name of their prophet, Herbert Armstrong). In 2009, long after the death of Mr. Armstrong, the leaders of the church held a very serious meeting with the result that they denounced his teachings as those of a false prophet and have embraced orthodox Christianity. For more details see here -
Grace Communion International. Formerly: Worldwide Church of God (apologeticsindex.org)
In the case of the SDA, Mrs. White wrote profusely with the result being that she taught many unorthodox doctrines, many of which have been covertly rejected over time by church leaders. One example would be the doctrine of the divine trinity. For many years the SDA rejected the trinitarian understanding of God, which resulted in their identification as a cult. Today, the SDA has embraced the trinitarian doctrine.