Juliet foreshadows Romeo's death in her first soliloquy (or "alone-speech"). Juliet is the longing, lover, praying for time to speed by so that she will be reunited with her true love. In part of the speech she unconsciously alludes to Romeo's death saying
when he shall die,
Take him and cut him out in little stars,
And he will make the face of heaven so fine
That all the world will be in love with night(25)
And pay no worship to the garish sun.
Note that even when talking of death and morality the language is infused with beauty and romance: "that the whole world would be in love with night"--that's wonderful language.
When the nurse comes in with news of Tybalt's murder we are also temporarily fooled, as Juliet is, into believing that Romeo is dead. That doesn't last long, however, since we learn of the true circumstances.