True prophets will attest to the humanity and divinity of Jesus Christ. By their fruits ye shall know them. But the general tendency of the life must be in harmony with the Word of God. Ellen White lived before the public for over 70 years. She was not perfect, but there was consistency and harmony in her life with what she believed.
Additional evidence is found in laws of Greek grammmar.
When there are two substantives and the second is in the genitive case without an article, for either substantive, the quality of the one is attributed to the other. The final Biblical test of the prophet is fulfilled predictions. Bible prophets gave prdictions that daelt with the main themes of the great controversy and salvation, not mundane things such as winning elections, astrology, etc. If the prophet predicts the future, it will come to pass. If the prophecy fails, the prophet would not be genuine.