Know the label before you shop. Dolce & Gabbana (also known as D & G: Dolce & Gabbana) is different from the lower-market D & G brand. Labels only come in black and white, and no holograms should be present. If someone tries to get cute with a multi-colored label, the bag's a fake.
Look at the ampersand for the real deal. A real Dolce & Gabbana logo has a slightly dropped lower loop on the ampersand, creating an uneven bottom justification. Spacing between the letters and ampersand is also tight.
Examine the authenticity tag under bright lighting for a few telltale features. On an authentic Dolce & Gabbana bag, the print is composed of small dots and is slightly raised. On a fake, the print is smooth. Printing should, once again, be a bit squeezed together and compact.
Feel the fabric or leather of the bag. A Dolce & Gabbana bag is expensive and crafted of the finest materials. Leathers are buttery soft, and fabrics are silky smooth. Cheap nylon, polyester, plastic or acrylic have no place in the authentic Dolce & Gabbana universe. This holds true for both the lining and body of the bag.
Shop smartly and skeptically. Dolce & Gabbana sells in stores like Neiman Marcus and Saks. No seller of authentic Dolce & Gabbana will sell for cheap or vend in a relaxed environment like a flea market or street stall. If buying secondhand, only go through a seller with a longstanding reputation and good feedback. EBay is a hotbed of fakes, and it's probably a good bet that an auction Dolce & Gabbana is counterfeit.