If you take in-game mechanics into account, everybody knows exactly what's in your stash or backpack the moment you talk to them, because quest items are detected immediately by the NPC in order to trigger extra dialogue or quests / progress.
If we go by their world, then maybe one of your group accidentally mentions the pair because of the absurdity regarding how they obtained it. Dallis could also be mad and mentions it in passing after using her purging wand / source vampirism spell as she leaves. There could also be a diary in Orivand's room writing about how he obtained it. There's many ways for the player character to figure out that it's hers, and we're likely supposed to fill in that blank ourselves.
Of course, the developers themselves could also have their explanation as to how the player knows that it is indeed Dallis'.
... haha, look at us having a serious discussion about a fictional woman's underwear.
