Other examples of foreshadowing in "After Twenty Years" include one very significant one: When the man in the doorway lights a match and the policeman sees him, there is a scar on his face and "His scarpin was a large diamond, oddly set." While the diamond's being large is a clue that the man is not a gentleman with refined taste, the fact that the diamond is set "oddly" indicates also that no reputable jeweler made the tie scarfpin. This observation by the policeman, who is actually Jimmy, of course, is Bob's undoing.
Another indication that something is going to happen that will not be what is expected is the description of the weather after the policeman walks away:
There was now a fine, cold drizzle falling, and the wind had risen from its uncertain puffs into a steady blow.
So often weather is used by authors as foreshadowing since it can so easily suggest mood and emotion.
Hope this helps!
I only see one instance of foreshadowing in this story. It happens very early on in the story.
In the first few paragraphs, Jimmy Wells (although we do not know that it is him yet) is walking his beat. He is pretty carefree, twirling his club and walking impressively. But then he sees something and he sort of goes on the alert (this is in the third paragraph).
What has happened is that he has seen Bob in the doorway and is suspicious of him. Bob quickly reassures Jimmy that all is well.
To me, this is foreshadowing. Jimmy's suspicions are really right on the money and Bob really is a bad guy. Jimmy's suspicions foreshadow the later action where Bob really will be arrested.