What elements or clues foreshadow the ending in the short story "After Twenty Years"?

The ending is foreshadowed by Jimmy’s deliberateness and Bob’s facial features.

O' Henry uses characterization to foreshadow the ending of the story.  The character traits of Bob and Jimmy that he introduces predict the ending.  

Jimmy and Bob were really good friends once.  They agreed to meet each other twenty years into the future.  Each managed to make the meeting.

Jimmy was a police officer.  The fact that he was unable to arrest his friend Bob is foreshadowed by the care and deliberateness to which he walked down the street.

 The policeman on the beat moved up the avenue impressively. The impressiveness was habitual and not for show, for spectators were few. The time was barely 10 o'clock at night, but chilly gusts of wind with a taste of rain in them had well nigh depeopled the streets.

Just by how Jimmy walks down the street, trying doorknobs and keeping an eye out, you can tell that he takes his job as a police officer seriously.  For Jimmy, being a cop is not just a job.  It is a way of life.  He would never break the law, even to help a friend.

While the foreshadowing of Jimmy is mostly about his personality, Bob’s role is foreshadowed through his physical features.  This is important, because it is recognizing Bob is what makes Jimmy choose.

The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow. His scarfpin was a large diamond, oddly set.

Jimmy recognizes Bob as a criminal from Chicago, and that is apparent from this scene.  The fact that he recognized him and knew that he was a criminal, but told no one, foreshadows the ending scene where he has another policeman arrest him. 

When Bob says that Jimmy is a standup guy, it foreshadows the fact that he will do the right thing.  Yet he also says that they are good friends.  This fits with what happens.  Jimmy sees Bob, passes him without introducing himself, and gets another policeman to arrest him.  Bob is his friend, and Bob is a criminal.  Jimmy can’t let him go, but does not have the will to arrest him himself.  For the sake of their old friendship, he has someone else do it.