"Francis doesn't know and can't declare her one"
Speaking in terms of unaided human knowledge, you are perfectly and exactly correct.
None of us would have any way to know for sure, were it not for God providing a sign from heaven. #43 --- check thou it out.
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There is one thing alone that makes one a saint - and it is not miracles - or the Egyptian magicians who replicated Moses' signs would also qualify as saints.
That one thing makes every believer a saint from the moment of believing faith. It is entrusting yourself to Christ and His sacrifice alone - with no self-effort, miracles, works, etc.
His Righteousness is transferred immediately to the entrusting believer at the moment of salvation. What the new believer's self-efforts could not do to earn righteousness no longer matters. God imputes the righteousness of Christ, baptizes them into the Body of Christ and gives them eternal life. From that moment, they are God's Holy Ones.
If the believer now has the life of Christ in them, it will be expressed in the quality of life they lead. To the degree they live a life of good works, they will be rewarded when believers go before the throne - or they will suffer loss, though they themselves will be saved.
To teach otherwise is to add to Scripture.
I dearly hope and wish that Teresa had entrusted herself to Christ alone for salvation before her death and truly was a saint. If she did not, there are no number of works, miracles, prayers nor declarations that can ever make here one now.