One of the great aspects of this story is that there is so little foreshadowing, that the ending is so unexpected. In retrospect, however, the reader can find small little clues that the author leaves to foreshadow the ominous reality of the lottery.
The first clue occurs right at the beginning of the short story as the boys:
"made a great pile of stones in the corner of the square and guarded it against the raids from the other boys."
Jackson also mentions that:
"Bobby Martin has already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones."
The inclusion of these details makes the reader ponder the purpose of the stones. Most readers would not come to the conclusion that the boys would use the rocks to kill someone, but would probably conclude that perhaps they were going to go skip rocks down at the lake or something.
The ritual aspects of the lottery process also suggest something unusual, the fact that the box is locked in the coal company office coupled with the proceedings where the official leads a chant.
The nervous reactions of the crowd to the drawings also make the reader sense that the lottery holds some darker meaning or purpose. For example, when the Watson boy draws for the first time, he "blink[s] his eyes nervously" and a few voices in the crowd reassure him: "Good fellow...glad to see your mother's got a man to do it."