Chekov's Gun:
I look for clues in movies now - if that book or car or gun or {insert name of item} is shown in closeup or shown more than once, you can be sure it will play a part in the plot. But since movies are visual, that is an easy clue to find.

Same thing applies on stage, if there is a prop prominently used or displayed, it more than likely will be part of the story.

Sometimes in writing the author has to be more clever. Or should be more clever.