IIRC, a Rabbi that makes Kiddushin takes a great responsibility (forgot the exact wording of the Gemmorah), at most he can be allowing Arayot, at least he can make the blessings said in vain.
As the two witnesses have no obligation of knowing the couple at all, and there's no obligation of asking the audience if they know anything incriminating or being an obstacle to the marriage, what facts does the Mesader Kiddushin Rabbi have to check Halachicly and to what extent, to ensure that no transgressions are made?
- What are their real names - is it enough to trust what they say (obligatory for the Kettubah)?
- Are they Jewish at all?
- Are they Halahicly a man and a woman - is it enough to trust their looks?
- Isn't the woman married?
- Are they related?
- Is he sexually handicapped - is it ever checked?
- Is there a Cohen/divorced situation?
NB, In [the State of] Israel the chief Rabbinate is responsible for keeping tracks of all the Jewish population and their statuses, but what does the Halachah for the rest of the history/world say?