There is solid evidence for miracles at Fatima, so apparition could not have been fake. One would have to show some heresy or falsehood being put forth by the apparition in order to prove it daemonic.
The cults succeed quite well in turning folks away from Christ by seductively dismissing, minimizing, directing away from or trashing HIS DEITY.
They can quite skillfully use truth, half-truths and bits of truth to accomplish that.
Fatima succeeded in the priority goal of the enemy on such scores by glorifying the Ashteroth-Mary-Goddess caricature INSTEAD OF JESUS THE CHRIST.
As Scripture makes very clear, HOLY SPIRIT AND WHATEVER HOLY SPIRIT DOES GLORIFIES CHRIST.