Her first assumption is that Romeo has been killed.
They both learn their respective identities from the nurse: Romeo first, then Juliet.
The nurse first finds Juliet dead
The Nurse tries to awaken Juliet.
The nurse finds Juliet dead first.
I'm going to assume that you're talking about Act 2, Scene 5. The nurse was slow to deliver Romeo's message regarding their marriage, and Juliet was impatient with her.
At first she suspects him of being duplicitous, of being other than he seems, but she gets over that.
The Nurse. :)
At first Juliet assumes that Nurse is speaking of her husband, Romeo's death other than the death of her cousin, Tybalt
Juliet's nurse.
If he is Mexican.
because the nurse is big fat and ugly and has no friends and orderes juliet around that is why she is angry at the nurse
The Nurse and Friar Lawrence knew about it before anyone else because the Friar performed the marriage and the Nurse was Juliet's close confidant so she told her everything.