The prologue and first act of Romeo and Juliet contain plenty of foreshadowing, but some of the most alarming examples come from Romeo and Juliet themselves.
The first time Romeo acutely foreshadows his and Juliet's fates occurs in Act 1, Scene 4:
…for my mind misgives
Some consequence, yet hanging in the stars,
Shall bitterly begin his fearful date
With this night's revels and expire the term
Of a despised life, clos'd in my breast,
By some vile forfeit of untimely death. (I.iv.113-118)
In these lines, Romeo describes "some consequence" that will result in "some vile forfeit of untimely death." Of course, the audience is aware that these lines are foreshadowing Romeo's own death, as the prologue told the audience of the star-crossed lovers' fates. Romeo also touches on popular imagery that occurs throughout the play, such as night, light and the stars.
Romeo isn't the only one who foreshadows their doom. Juliet also has an eery line when she first sees Romeo:
Go ask his name.—If he be married,
My grave is like to be my wedding bed. (I.v.143-144)
This line could be written off as hyperbolic teenage girl speak, but the line comes across differently since the audience knows the play's end. Shakespeare seems to love foreshadowing Romeo and Juliet's demise, as there are plenty of delicious examples like this within the play.