It depends on whether you talk about Sorcery as the literal definition of "exercising supernatural powers with help from evil spirits" or if you are discussing more specifically what we consider to be acts of Sorcery today (such as hexes/juju etc.). From my Western background, what i understand Sorcery to be is the type of magic that 'magicians' (the type found in fantasy novels/movies) perform which includes things such as summoning fire/lightning from nothing, levitation of items, shape shifting etc.

I don't believe that people can use powers given to them by evil spirits, but i do think it is possible that some rumors of 'sorcery' and perhaps even Sorcery as i have been brought up to understand it, have some truth behind them.
Some things that are considered Sorcery probably have perfectly logical and scientific explanations, we simply aren't advanced enough in the adequate field to understand how they work.

As far as proof goes, and the ability to replicate what has happened (i'm talking about what we perceive to be Sorcery, as i said, i don't think people are joining forces with evil spirits). Perhaps this again is a case of us not being advanced enough to understand how to recreate it (even the people claiming to be practicing sorcery or magical acts).

A simple way to explain what i mean by this, would be to look at a hypothetical recreation of the discovery of fire. Most likely, the ability to create fire was discovered purely by accident.
Perhaps a caveman accidentally dropped a piece of flint onto another piece surrounded by dry grass. Everyone with him would have seen the grass burst into flames, which would (at the time) probably have looked like some crazy supernatural magic. When they met another group of people or got back to the settlement they would describe what they had seen (if their language was advanced enough for that). Then of course those who hadn't seen would ask for a demonstration.
Perhaps the cavemen did not understand the difference between flint and normal rock, or hypothesized that it was that specific rock that was dropped into the grass rather than any flint being dropped onto the other rock that caused the fire. Even if they set everything up exactly as it was when fire was made, the chances that the caveman would hit the rocks together in exactly the right way (especially as he would most likely assume that the flint needed to be dropped onto the second flint not just struck together) to create fire again are slim at best. Therefore even though the caveman had in fact created fire the first time, which was seen by few and would have been considered Sorcery by them, could then not be recreated on demand to others, leaving exactly the same situation we have today with many things we consider to be Sorcery; A 'Sorcerer', a small group of people who witnessed the sorcery and swear that it is real, and a large group of people who haven't seen it so are skeptical at best.

Of course today we know exactly what fire is and what causes it and can create it on demand (unless our matches are wet or our lighter is out of gas, but even in that case we can explain why it isn't working).

When we consider the magic of Wizards (i know i'm stepping into very fantastical waters here, but bare with me), who is to say that it isn't possible to harness energy sources found in non-visible dimensions which we don't have the technology to harness or even identify yet, to appear to create a fireball from nothing or cause an inanimate object to levitate? But that is just a side thought, perhaps wishful thinking .

Ah, thanks for clearing that up for me, in which case then i probably do believe it to be possible (or at least really like the idea), and that we simply haven't discovered a power source capable of allowing an individual to perform acts of 'sorcery'.

I was slightly confused as a lot of what was being discussed sounded like practices such as Voodoo and Witchdoctoring in which case i don't believe it to be 'magic' or even consorting with evil spirits. Although i do think they can have some powerful effects on people who believe them to be true.