Which game is the network going to show when they give two options?

I don't understand when a TV network will have a block of time set aside for a ball game and when I check to see who's playing there's a choice of two different games. I live in Tucson and MLB Network is showing game at 7:00. It's either going to be the Rockies vs. Dodgers or A's vs. Yankees. How can I know which game will actually be shown without having to wait until 7:00?