(Jetlag has its good sides, too...)
In Inception (a movie about which you'll read a lot on this blog), there is the concept of a 'totem', an idea that provides "an elegant solution for keeping track of reality", as one character puts it. It's a small physical object which you would carry with you, and which has a particular behavior known only to you. For instance, one character has a loaded dice as his totem, the behavior of which only he knows. The function of this device is to tell you, "beyond any doubt", whether you are in the real world or within a dream world. From the behavior of your totem, you could tell the difference.
Now I'm not quite sure what to think of this idea. If you are training for lucid dreaming, one of the techniques you employ is habitual reality-checking: you constantly ask yourself whether you are dreaming or awake right now; and one of the ways to tell is to look at a string of symbols (a multi-digit number, such as on a digital watch, or a line of text), which generally isn't stable in dreams. The idea of a totem in the film, however, seems not to be like that. (Those reality checks aren't reliable enough to fulfill the crucial function assigned to totems in the movie.)
Perhaps it rather has to do with another concept in that movie's world, that of 'shared dreaming'. In Inception, all the dreams are arranged so that several persons are in the same dream world, acting and interacting within it, somewhat similar to what happens in a multi-player video game. Since only one of the people sharing the dream 'hosts' the world (sometimes referred to as 'the dreamer'), it's up to that person to build up the physical surroundings in it. But this means that if you are in someone else's dream, that person has created the physical world around you, and thus when you use your totem, it's the host who would have to arrange for it to behave in its peculiar way. But the host doesn't know exactly what that way is, and so your totem doesn't behave as it should, showing that you are in someone else's dream world.
If that's the idea, however, it doesn't seem to withstand a closer examination either, even within the rules that make up the world of the movie (remember: we're still talking about the fictional world of Inception, not about the real world, i.e. our world).
For one thing, I can't quite see how it would prevent you from mistaking your own dream for reality, if the outline I have given above is correct. For another, it doesn't fit with a claim made earlier on in the movie, namely that most of the details are filled in by those experiencing the dream world, in order to "make it seem real". And moreover, remember the business model of the group around diCaprio's character? It's about stealing secrets (called 'extraction' in the movie). The idea is to build a secured area into the dream world which then is filled by the victim's subconscious mind with some valuable secret information, and that's what they then steal. But if it's possible to 'extract' even such well-protected secret information from someone, then what would be different about the information how your totem is supposed to behave? What would prevent one from stealing that bit of information, and then fake the totem's behavior? So, if totems worked the way I've sketched above, they wouldn't be good enough to fulfill the function ascribed to them by the characters.
Still, the concept of a totem is an interesting idea, a clever solution to the problem of distinguishing between dream and reality (keeping in mind all along that 'reality' here means the world of the movie, which is called 'real world' in the film, but is still a fictional world). Again, this is an instance of the question of where the borders between reality and unreality run, and the way it is solved here suggests that the person who experiences both reality and unreality has itself some crucial role to play here, as have the experiences themselves.
There is an old story (I don't remember where I originally read it) in which a man is haunted by what seems to be the ghost of his late wife. She appears frequently and seems to know everything about him, even his most secret thoughts. Disturbed and not quite sure if she really is an omniscient ghost or merely a figment of his imagination, the man consults a Zen master, who advises him to put a handful of beans into a bag and ask her, on her next appearance, to tell him the exact number of beans within it. If she can't do that, then the man would know he's only dreamt her up (and presumably get rid of her by virtue of that realization).
The strategy here is basically the same: create some privileged knowledge (or non-knowledge) which you can be sure nobody else can have, and test it in order to find out whether you're in the real world or not. In the beans story, the ghost's ignorance about the number of beans demonstrates her to have implausible limits to her 'omniscience', which thus turns out to be fake. In Inception, your totem supposedly can assure you that you're not in a dream. But unfortunately, elegant though this "solution for keeping track of reality" may be, it's ineffective outside the fictional world of that movie. (Just in case you were in doubt.)