Of course Ellen White knew she had the Spirit of prophecy
Then I fell down at his feet to worship him, but he said to me, "You must not do that! I am a fellow servant with you and your brothers who hold to the testimony of Jesus. Worship God." For the testimony of Jesus is the spirit of prophecy (Rev. 19.10).
When God raises up a prophet, we need to fully investigate that, and if true, we should heed that prophet. To do otherwise is very foolish.