How do Catholics make sure that the supposed saint they are asking intercession of is truly saved? The doctrine of the intercession of saints states that deceased saints can be asked (via prayer) to intercede for the living. This assumes that there is:

A living Christian.
A deceased saint.
A one-way communication from the living Christian to the deceased saint. (*)
A two-way communication between the deceased saint and God.

(*) I say one-way and not two-way in point 3 because the deceased saint does not communicate back to the living Christian (or at least the doctrine doesn't state that possibility as far as I'm aware).
My question is regarding how Catholics make sure that points 2 and 3 are really taking place. I imagine that not everyone that one decides to pray to is necessarily a deceased saint. This is easy to see with an extreme example. Let's say someone decides to pray to Adolf Hitler. Most would agree that Hitler is very likely not saved. So praying to him would be a wasted prayer, because Hitler, though he meets the condition of being deceased, doesn't meet the condition of being a saint. So that's an easy one to tell, but what about other, more ambiguous cases? What if my uncle Bob, who passed away last year, was a believer, was very virtuous but also had his fair share of defects? How  can I tell if he was saved or lost at the moment of death? And what about deceased virtuous pagans who, though they were not strictly Christian, still had a chance to be saved and, therefore, to intercede for the living?
In other words, I would like to know if Catholics have a standard protocol in place to determine which deceased persons are trustable candidates to ask intercession of, to avoid false positive situations in which they accidently pray to someone who was not truly a deceased saint. Similarly, are there any negative consequences that might occur if someone accidently asks intercession of a deceased non-saint (e.g. someone prays to a deceased relative who is actually in Hell/not saved)? Is it possible that a demon could take advantage of the situation and answer the prayer instead, pretending to be the deceased non-saint?
Unless the saint has been canonized by the Church, one cannot know for sure whether he or she is truly a saint or is in purgatory or hell.
One must pray for the deceased (suffrages for the dead: &quot;Masses, prayers, or acts of piety offered for the repose of the souls of the faithful departed&quot;), in case they are in purgatory. Once they are in heaven, then they can pray for us.
Fr. Réginald Garrigou-Lagrange, O.P., Life Everlasting: A Theological Treatise on the Four Last Things: Death, Judgment, Heaven, Hell, ch. 26 &quot;Charity for the poor souls [in purgatory]&quot;:

May we also pray to the poor souls [in purgatory]? The liturgy does not pray to them. But we are not forbidden to pray to them, though we must give preference to prayer for them. Here is a sentence from St. Thomas: &quot;The souls in purgatory are not in the state of praying, but in the state of being prayed for.&quot; [Summa Theologica II-II q. 83 a. 11 ad 3]