In the book, all the main characters (except Zachry) have a birthmark below their left arm resembling a comet that each character notices the preceeding character possesses. In the film, all the birthmarks are in different places and appear on all the main characters at least once throughout the movie:
Adam Ewing's birthmark can be found at the start of the film when he faints in the sun. Dr. Goose unbuttons his shirt and finds a key around his neck. As he does so, you can see the birthmark on his chest.
Robert Frobisher's comet is on his back which is clearly visible when we first see him in the hotel room with Rufus Sixsmith
The easiest to spot, when in the elevator with Sixsmith, she removes her jacket and Sixsmith remarks about her birthmark and that a close friend of his had one similar to that.
As he recounts his night with Ursula as a young man, his birthmark is visible behind his leg as he enters the bed.
As Dr. Ovid removes her fabricant collar, her birthmark is found on her neck and be seen throughout the film also in the same place.
When Zachry and his granddaughter are looking up at the planets in the sky at the end of the film, his birthmark on the back of his head is focused on.