To discuss foreshadowing in Tom Stoppard's Rosencrantz and Guildenstern Are Dead, the first question to be asked is whether you've read Hamlet, which Stoppard took the two characters from. It's important to point out that the play is a sort of experiment on Shakespeare's classic tragedy. Stoppard wanted to see what he could do with two characters from a play outside of the play itself. What he came up with is an absurd exploration of the meaning and function of literary works in general. If you know Hamlet, the foreshadowing doesn't really work as foreshadowing, because you know they're going to die. If you don't know Hamlet, and perhaps if you've ignored the title, you may have to look a bit closer for the foreshadowing.
From the beginning, an impending sense of some inescapable inevitability overshadows the dialogue between the two. The coin toss repeatedly landing on heads, and their futile attempts to call probability into play, suggests there is a force at work that gives them no choice. The repeated descriptions of being awoken before dawn, and the exploration of these moments as their furthest memories, gives a further sense of dark mystery. Was the man on the horse the angel of death?
The Player's arrival can be compared to the Greek chorus, in that he offers commentary on the events of the play while acting as a part of the story. He repeatedly reminds the two characters that, in plays, death is the common end, and with the play's extreme self-awareness, it isn't an intellectual stress to realize this is clearly a play.
The play continues to revisit these same few tropes: the two protagonists try to take control of something they have no power over, the Player once again tells them that tragedy ends in death, and the character dialogue and segues into scenes from the actual Hamlet reinforce the idea that the play is a piece of literature making fun of how literature works.
So, looking for foreshadowing, consider the Player's commentary on tragedy and performance, look for ways in which a sort of wrongness with the world hints at the doom they meet in the end, and at least read a summary of Shakespeare's tragedy that inspired this comedy.