Don't worry: Your body will give you more than a few signs you're dead. After that, things gets sketchy. There are countless films, books, and television shows about what might happen after death. Sometimes, deceased characters don't realize they're dead because they're trapped in purgatory - an intermediate plane between heaven and hell.
So how the hell do you know you're in purgatory? You have to pay attention to certain omens. Maybe you plunged into purgatory unknowingly, but you don't have to stay in the dark forever. In fact, if you ever wish to accept your demise, you have to be able to recognize what happened. Here are a few clues.
People typically check their reflections a few times a day. If, one day, you look into the mirror and no one is looking back, you should be alarmed. Usually, only vampires lack reflections because they're trapped between life and death.
So if you can't see yourself in that store window, you're probably dead.
Some Catholics believe purgatory is a place for spiritual refinement. To them, it exists for believers who need a little time to cleanse themselves more thoroughly before entering heaven. So if your body was in purgatory, it wouldn't need to improve, change, or tan. It would likely be resistant to all physical alterations.
If you land in purgatory for a period of spiritual cleansing, memories of your old life will probably need to be erased. After all, one can't enter heaven still plagued by every time they lied or stole a piece of gum. Though it won't happen immediately, those in purgatory are likely to lose every memory of their old life, especially the naughty ones.
A fading memory is a great indicator your life has already ended. Even if you aren't dead, the loss of memory can imply you're close to death.
Part way through the film, The Matrix, Neo starts seeing glitches in his world. He notices the same cat multiple times. And in extreme instances, things like windows turn into solid brick. Neo isn't exactly dead, but he's in something like a virtual dream world.
With advances in computer technology, it may be possible to download someone's consciousness into one of these faux worlds in real life. If that happened, it would be incredibly difficult to tell if you were alive and fully functioning or in some sort of computer simulation.
If you begin to experience frequent glitches or bouts of déjà vu, chances are you're dead.