If it's significantly (ie more than 10/20%) cheaper than the full price, then it's almost certainly fake.
There was a thread a few months back with links to some very convincing websites that appeared to sell genuine products at about 30% of RRP, but were fake.
Some people have bought fakes that appear indistinguishable from the real thing, but on the other hand, quality and durability could be poor, and you'll find there's no chance of a refund, or not worth posting back to China at your own cost.