Catholics. Praying to Saints?

Yes, the Church does make a judgment on whether or not that particular person is in Heaven. It's not infallible, though, because it's not about faith or morals. There is definitely the possibility that we could be praying to saints, especially older ones, who are not actually in Heaven. However, we're only passing along prayer intentions after all, so it's nothing that damns us. Plus, because God is omnipotent, He hears those prayers as well, so they aren't wasted in any case.

The Church does try to take some precautions regarding canonization of saints, for the same concerns you mention. First of all, all saints have lead good lives, as far as the Church knows; the scenario you suggest is possible, but very unlikely in my opinion. Secondly, part of the canonization process nowadays involves the verification of at least two miracles attributed to the saint, at least one of which must have been performed through his or her intercession after death. Given that miracles are God's work, and that God does not want to delude the faithful by performing a miracle asked of a false saint, we can infer that it's safe to call that person a saint.