It is very important to understand what an allusion is prior to finding evidence of one in a text. According to the eNotes site's Guide to Literary Terms, allusion is
a reference, usually brief, often casual, occasionally indirect, to a person , event, or condition thought to be familiar (but sometimes actually obscure or unknown) to the reader.
Shirley Jackson, in her short story "The Lottery," she makes one very prominent allusion through dialogue provided by Old Man Warner:
Used to be a saying about 'Lottery in June, corn be heavy soon.'
Here, Jackson is alluding to the fact that human sacrifices have been made throughout history to insure good yields at harvest. While some readers may not pick up on this (given the allusions are "sometimes actually obscure or unknown"), the fact remains that human sacrifices have been performed in the past for crops.
This allusion also provides additional foreshadowing. Other foreshadowing from the text comes from the second paragraph of the story:
Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones.
While an allusion is not made here, the clues are evident when used in connection with Old Man Warner's statement.