It depends. The way I like to do it - or at least the way I like to think I do it - is that the further away the event, person or whatever is being foreshadowed, the more vague I be, and the closer, the more obvious.
And that seems to work so far for my novels, but if you were doing, say, a mystery novel, then you need to have different pieces (so to speak) that fit together at the end to form the big reveal, such that reading it a second time, the person can go, 'oh yeah! that makes sense now. That's why such and such happened.' But one thing you have to watch out for is the way you reveal information. Say you have your character mention an object in the room offhandedly. If your character doesn't have a habit of doing things like that, and you don't have dummy objects that they mention that don't have any effect on the plot, your reader is just gonna go, 'why did they mention that? It must be important.'
Your readers are most likely going to assume that everything you've written, you've written for a reason, and they might read into that - unless you give them reason to believe otherwise. False information, irrelevant information etc are all things you should pepper in occasionally. One thing I've found in a lot of novels is that 'the hero is always right'. What I mean is, whenever the protagonist guesses about something, is told about something, figures something out, they're never wrong. The information is immediately treated as fact. It's stupid, it doesn't make sense, and it's terrible for any sort of mystery or foreshadowing. The protagonist will guess what's about to happen and oh, what a shock! They were right yet again. Such a surprise that was!