The way you can tell if a prophet, apparition, “angel”, alien, or an entity back from the dead like “Mary” is speaking for the God of Abraham Isaac and Jacob is...
Their words will ALWAYS conform to and confirm the Bible.
This is because God, who "is the Word" cannot contradict Himself.
This is how we know that either Joseph Smith was a lunatic deceived by demons or not... (and we know he was deceived because his magic tablets or whatever contradicted the Word and tried to rewrite it as it “added a new covenant or agreement” to the New Covenant or new agreement or new contract or new testament of God.
On the face of it, there is no reason for “Mary” to come back from the dead to say anything. If her Son was indeed the Messiah, He did it all, He who is the Word made Flesh, said it all, and “it is Finished”, a finished work upon the Cross.
Outside of there being no extra-biblical need for Mary to come back from the dead to say anything to anyone about the Messiah or God the Father, what that demonic entity at Fatima said was this:
“Only I can help you. My Immaculate Heart will be your refuge and the way that will lead you to God.”
(Read it here for yourself... http://fatima.org/story2.html)
What the demon at Fatima said is in direct conflict with the Word of God that was spoken out of the mouth of Jesus Christ Himself!
John 14:6, “Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.”
Why would Mary, a sinner who acknowledged as much, who waited for the Spirit to fall at Pentecost with the other disciples, in obedience to Jesus Christ, why would she have a need to come back and say something on top of the finished work of Christ?
There is only one answer.
She wouldn't.
It isn't Mary.
The real Mary is appalled at the deceived sheep of the Catholic Church who dare to make her into an idol, and angry (as is her Son) at the works of Satan, which He came to earth to destroy.
If nobody should say anything on top of the finished work of Christ, there should have been no Christian books published after the conclusion of the Fourth Gospel. And this Religion Forum should fold, and we all just keep our mouths shut.
I know that’s not what you mean; nor am I putting words in your mouth. But that would be an unavoidable consequence of insisting that no one need say anything now, since Christ’s work is finished.
Mary’s words, in any case, are not divinely revealed doctrine. They are private revelation which does not demand the assent of faith. Any interpretation or implication that Mary replaces Jesus is obviously false.