How is a revelation affirmed?
If it's a healing, I believe the usual criteria are that it must be (1) medically inexplicable (2) instantaneous (3) complete and (4) permanent. So you need documentation and the testimony of medical esperts that the healing was not caused by drugs or other treatments; that it wasn't a mere gradual or partial improvement; that there hasn't been any relapse.
If it's a teaching, it must not be in conflict with any of the truths taught by the Catholic Church. Nor must it be a novelty acompanied by a demand that all must believe it under pain of sin. This is because we believe that God has already revealed everything we need to know for salvation.
If it's any other kind of phenomonen, the usual evidentiary methods and rules apply, that you would expect in a court (forensic) inquiry or in a scientific investigation.