SPOILERS AHEAD
Certainly a top that spins forever is an indicator you're dreaming, as its impossible in our physical reality. But within a dream, anything is possible, including tops that fall or don't fall.
The spinning totem is fine for proving to yourself that you're in a dream, but it's useless to prove that you're not. In fact, there's never any way to prove you're not dreaming. There's nothing you can say or do right now to convince anyone, including yourself, that you're certain you're not dreaming. As a fine example, some of you will read this, think about it, and, in fact, dream about it and in your dream, you'll hear yourself saying "I know I'm not dreaming!"
So, that being said... doesn't that negate an awful lot of a very necessary premise of the movie?
Dreams aren't 'areas'. It's a series of electrical impulses (electrochemical processing).
So basically; whatever you decide to believe about the 'totems' is totally fine, because this entire concept is just made up from nothing and has nothing to do with our understanding of what goes on in a human brain.
Or in other words; if this is sci-fi, then Bugs Bunny is sci-fi.
So on-topic; do you think Bugs Bunny will turn orange in the future from eating all those carrots (carrots actually contain properties that can change pigment, but not permanently)?
A totem's purpose is not just to identify whether you are in a dream or not. It is a way to maintain your own identity. Each totem is intimately known only by its owner and nobody else.
The deepest secret of Inception is revealed by recognizing that the top was not Cobb's totem. It was his wife, Mal's. Revealing the true dreamer also answers the question of whether it is a dream at the end.
The whole movie was not about a guy trying to resurrect his dead wife through dreams. It was about a wife trying to resurrect her dead husband.
Yea good point, in the sense that Plato talked about it is impossible to prove that anything exists outside your conciousness. However, philosophical stuff aside there is a very definite difference between what we understand to be dreaming and reality, that is to say we may not realise we are dreaming while it is happening but once we 'awake' and our brains are firing on all cylinders the absurdity's of dream logic become clear.
So I kinda thought the characters got into the habit of using totems all the time even when they knew they were awake, not to make sure they truly were awake, but so the habit of using them carries over to the dream world. So in the event of them forgetting they were dreaming (the danger of which was mentioned a couple of times)they might crack out their totem and with the nature of dream physics being as illogical and inconsistent as it is, realise they are dreaming again and snap out of it.
I'm pretty sure at some point at some point Christopher Nolan read up about lucid dreaming because the idea of the totems matches up pretty closely with 'reality checks' some people do to try and induce lucidity in dreams. For example someone may get into a routine of looking at their watch carefully or looking into a mirror, with the hope the habit carries over into dreams and when they do, notice the quirks like the time changing constantly or reflections behaving extremely oddly.
Could be complete bollocks but I thought I might give a stab at answering your question.