At the time of pouring, there was no secret mechanism in place to inform the particular participant who brought the wine being poured.
given the fact that (1) it was common knowledge (each one knows it, if each one knows that the others know it, if each one knows that each one knows that the others know it, and so on) that each grape-region combo was assigned to no one or one individual for each session, and (2) all participants were required to bring a wine of the assigned grape-region that they know very well and ensure it of a classical style most representative of the grape and region, the task of detecting self-brought wines becomes trivial to our participants, and therefore the informing mechanism stands.