Well, Chris Weitz seems to wat to keep the movei as close to the book as possable, and in the book, Bella first sees the wolves in the meadow with Laurent.
In the trailer Bella says "Jake! Run!" so I don't think she knows there.
Maybe she already did know there, but she didn't in the books, which tends to make me think that is the scene when she finds out.
He just phases to protect her.
I think that Bella will see Jacob in his werewolf form when he is about attack Laurent and right away know that it is Jacob,because in the movie trailer Bella sees Jacob and yells "Jacob NO!" when he jumps and turns in a werewolf. Or Jacob is going to make Bella guess that Jacob is a werewolf.
I agree with you when u say that she will know about it just when Jacob phases to kill Laurent..., maybe later, in another scene, he will explain her all the werewolf stuff, or maybe there is a bonfire party and she listen to the elders...