How can I know if this product is authentic?

A friend of mine, bought me a birthday present, a cologne Dolce&Gabanna. How can I truly know if its a true Dolce&Gabanna product and not a cheap copy of the product? These copies are really on selling at my place, so you cannot really know what you buy! Do you know any tips with which I could recognize a true Dolce&Gabanna product and a fake one?