What events in act 2 suggest that Romeo and Juliet end unhappily?
In the end of Act II, we assume a wedding is taking place, or at least the arrival of the bride and groom. In nervous anticipation, Romeo says:
Amen, amen! but come what sorrow can,
It cannot countervail the exchange of joy
That one short minute gives me in her sight:
Do thou but close our hands with holy words,
Then love-devouring death do what he dare;
It is enough I may but call her mine.
Romeo doesn't care if he dies immediately after their marriage as long as he has been permanently connected to Juliet for all of life. This is the most significant suggestion of an unhappy ending for the two that I can find in Act II. Act I and Act III have more distinct evidences of foreshadowing.
Throughout the rest of Act II, we see references to the idea that love kills in a few other places. In scene 2, Juliet says she'd kill him with her cherishing, and that her kinsmen would kill Romeo if they found him. In scene 3, Mercutio says that love has already killed Romeo on the opening page.