And I don’t think we are talking here about a Safek Yehudi. That is a whole different Shaala. The issue is that you know who is who, it’s just that you don’t know which one you hit.
In the case of the Mekudeshes the problem is actually the Krovim, since the Mekudeshes will tell you that she is MEkudeshes. As Tosafos says, otherwise nobody would be able to marry anyone since she might be an Eishes Ish. The Kerovim aren’t aware, so asking wouldn’t help.
Besides, it is easily discernible who is the Yid, even if it would take asking him who his uncle is. The Mekudeshes is out there and can’t be found; she is mixed into the world.
(What would you say about a group of non-Jewish Nimolim dressed as Yidden, and all it would take is saying, “Good Yom Tov. Du Hast Vu Tzu Esen”, to find out who is and who isn’t, wouldn’t you say that’s Kavua?)