In Chapter 29, what things foreshadow Mr. Ewell's attack?To Kill a Mockingbird by Harper Lee

One very strong hint of Bob Ewell's vindictive act is in
his insulting actions and words in front of the post office:  "Mr. Ewell approached him,
cursed him, spat on him, and threatened to kill him "if it took the rest of his life." 
The reports of this incident occur at the end of Chapter 22 and the beginning of Chapter
23 of To Kill a Mockingbird.

Also,in
Chapter 17 Scout narrates that Robert E. Ewell, "a little bantam cock of a man"--which
indicates his attitude--takes the stand and regards Atticus with "haughty suspicion." 
And, while Bob Ewell in on the stand during the trial of Tom Robinson, he is asked to
write upon an envelope.  Unknowingly, he complies, but when he looks up, he discovers
the judge staring at him as though he were "some fragrant gardenia in full bloom on the
witness stand," and the jury watching him.  Sensing that Atticus Finch has somehow
acquired an advantage over him. Ewell becomes angry and declares that Atticus Finch is
taking advantage of him.  This anger of Ewell's and embarrassment in front of the
courtroom crowd is not to be forgotten by Ewell. He tells the judge that "tricking
lawyers like Atticus Finch took advantage of him all the time with their tricking
ways." Even Atticus later mentions that Bob Ewell will not forgive him for exposing his
ignorance.

On the night of the attack upon the children by
Bob Ewell, there are foreshadowing of the danger.  For instance, Scout notes
that

a
solitary mocker poured out his repertoire in blissful unawareness of whose tree he sat
in...[the whippoorwill sang] Poor will, Poor
Will.

Like the mockingbird,
Scout is unaware of what will soon happen; she trips on a root growing in the road as
her costume impedes her movements.  then, when she and Jem turn off the road and enter
the schoolyard is "pitch black."  Scout tells Jem, "You should have brought the
flashlight."  And, Cecil jumps out at them, and asks them if they are not "scared of Boo
Radley?"  During the performance, little Scout falls ill. As she and Jem leave the
auditorium, "it was still black dark," but they walk toward home even though Scout has
trouble in her cumbersome costume.  When Scout feels pressure on her neck, she thinks it
is Cecil teasing again as they hear a sound like leaves blowing in the wind "only there
wasn't any wind, and there weren't any trees beside the big oak behind them.  At this
point,they are attacked.

Later, in Chapter 29, Scout
remarks,

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Somehow, I could think of nothing but Mr. Bob
Ewell saying he'd get Atticus if it took him the rest of his life.  Mr. Ewell almost got
him, and it was the last thing he
did.

Those remarks of Bob
Ewell's coupled with the dark, sinister night and the haunting sounds of the mockingbird
and the whippoorwill, certainly suggest that ill is going to befall the
Finches.