It’s pretty clear from Tosafos and the Sugya in general that the only reason you don’t know is because you didn’t see.
It can be beard, Peyos, clothing, shoe straps, simple recognition, or perhaps even a five minute conversation. This is the issue, that had you been there close up you would know who is whom.
In the case of Keddushin the Krovim aren’t aware, and more importantly, there is no place to go to see the Issur and the Hetter.