There are loads of links here that will instruct you how to spot a fake and a bunch of websites out there. I don't have any of the links saved and don't have time to search for you but it's out there. But when it comes down to it, if the person making/selling the fakes knows what they are doing, the only way to tell is by smoking it, presuming you have smoke more than a few of those before.

But - if you have some guy trying to sell you macanudos as cubans, I bet you real money that everything he is selling is a fake.