Regarding the intelligence of viewers, you’ve stumbled upon the great mystery of art. In short, no, they are not smart enough and yes, they are. Both answers are correct because there is no such thing as a “viewer.” There are only people who watch movies/read books, and each one brings unique levels of engagement, commitment, and interest to a story (not to mention other factors such as environment, multitasking, etc). As writers, we all hope for the Ideal Reader (see Nabokov’s opening essay at the beginning of Strong Opinions).
Regarding foreshadowing specifically, it's helpful to think about it in several different ways. Doing so will help you strategize as you build and edit your story.
- As obvious clues to an outcome. Think of Chekhov’s rifle over the mantelpiece. If there is a rifle on a mantelpiece, it will go off by the end of the play. This is foreshadowing for the observant/expectant viewer.
- As a pattern visible to a few people either during or after viewing. Think of Scorcese’s use of Xs behind characters who were slated to die by the end of his film The Departed.
- As easter eggs for the repeat viewers. Think of some of the details in Fincher’s Fight Club: the spliced-in bits of Tyler Durden into some scenes (implying Jack’s madness), and the payphone near the beginning that does not accept incoming calls. These are details that a viewer can decide have meaning or not (the former is open to interpretation, the latter is a specific clue).
- As strategic details that add texture and depth to a piece. Think of the Christopher Nolan’s The Prestige and the drowning death of Angier’s love as foreshadowing the death of his own copies. The foreshadowing adds emotional depth to the revelation at the end.
Ultimately, foreshadowing as a technique or strategy depends a lot on the audience’s attention to detail and their commitment to active thought during a viewing. I suspect that many people miss foreshadowing that isn’t obvious, so it isn’t necessarily a good clarification strategy. Rather than looking for ways to simply foreshadow something to assist the audience, think about how editing or color grading styles can affect perception and create a pattern that is easily interpreted (a good example is The English Patient; a complex example is Lynch's Lost Highway).
As a writer, you can add foreshadowing after completion of the script/story or as you go. Ultimately, though, foreshadowing relies a lot on consistent use of symbols as well as an observant, committed audience. That being said, it can create a lot of satisfaction for viewers who recognize the foreshadowing as it occurs or on a second viewing.