If you read a book, say by Hemingway, and a gun is mentioned in the first few pages we can surmise that someone will die by the end of the story. It's foreshadowing, and authors have a host of means to hint at or foreshadow the death of their characters.
Life, however, isn't usually so tidy as a novel. Which hasn't stopped people from arranging a host of superstitions which foreshadow the death of someone near or within the family. Take, for example, the Irish Banshee, the sight or sound of which foretells the death of someone near and dear. Or the Barguest, who if he doesn't directly cause your death may foreshadow it. Then there are the hosts of superstitions (some actually remarkably commonsensical) that virtually guarantee that no death goes unforshadowed. A few of which I've listed below:
- A live adder on the doorstep (I'd argue this is common sense, get rid of the adder and you'll probably live longer)
- If your dog becomes rabid it foretells the death of someone in your family (if your rabid dog bites someone it almost guarantees it)
- Fleas leaving a body foreshadow the death of the host (possibly a sign of ill health)
- Rats leaving your house (someone inside will die)
- 3 knocks heard on the door, then when answered no one is there (Usually means someone close to you has already died)
- A beetle walking over your shoe (foretells your own death)
- A broken clock suddenly chimes
- A single magpie circling the house, a dove circling, an eagles cry or a jackdaw settling on the house are all omens of death
- A bird (especially a robin) flies into the house (foreshadows death in the household)
- A Nightjar or Whippoorwill heard after dark foreshadows death
- A dog howling three times in the night or early morning
- To dream of birth or hares foreshadows death
- A white moth in the house or trying to enter or to see a butterfly at night foreshadows impending death
- If 13 sit at a table, one will be dead before the year is out
- If there is a corpse in the house all mirrors should be covered, otherwise anyone who views himself in one will die
- To point at a funeral will cause one to die within a month
- A funeral on a Friday will mean there's another death within the family before the year is out
There are of course many, many more and this is but the briefest of lists.
Read more here or start here on Wikipedia (sorry, they're not so tidily arranged but a bit of searching will lead you to a few).