It is really easy. You find the dairy code printed on your milk bottle, enter it at the website (the search box is prominent on every page hosted by the site), and viola! find out what dairy makes your milk and where it is located.
I found this site a few years ago and thought it was neat. But when I realized that milk tastes different depending on where it is from, I decided to find out which dairy I like.
So far the verdict is that I like milk from Berkeley Farms in Hayward (they make milk for both the Berkeley Farms and Hy-Top labels). And I don't much like the milk from Super Store Industry in Fairfield (Bayview and Sunnyside labels).
I wish I could drive out to Winco everytime I need more milk. I wonder if Savemart would change dairies? Probably not. I think the dairy I like is over 40 miles away.
Where is your milk from?