Not all ugg-style boots made by companies other than UGG Australia are knockoffs -- some are high-quality (or inexpensive but similarly styled) boots that were made legally by other companies. However, the UGG Australia brand has become such a moneymaker that there are plenty of shady manufacturers trying to capitalize on the label's popularity (and high prices) by selling cheaply made, lower-priced knockoffs as though they're actual, brand-name UGGs.
Web sites selling faux UGGs often use photos of actual UGG boots, which can make it difficult to tell whether you're ordering a counterfeit version. But once you've got a pair of boots in your possession, there are telltale differences. If your boots are made with real fleece-lined sheepskin, you won't be able to separate the fleece from the suede -- at least not without the help of sheep shearer. Knockoffs are often composed of foam sandwiched between cowhide on the boot's exterior and fleece on the interior [source: Kemp]. To find out what your UGG (or counterfeit UGG) is made of, grasp the inside of the boot with one hand and the outside with the other, and attempt to pull them apart from each other. If you're able to separate them, you're probably holding a fake.
Because much of the UGG's trademark stitching is visible on the exterior of the boot, it offers another way to spot a fake. If the stitching is uneven, if the seams are crooked or if some stitching is missing where the seams connect, your product is probably counterfeit. The same is true if the sole is inflexible; authentic UGG boots feature a thick but pliable sole.
According to UGG Australia, authentic UGGs include three security features that set them apart from faux UGGs. First, there is a reflective sticker depicting the company's sun logo on the exterior of the shoebox. And, when turned to a 90-degree angle, the logo will change from black to white. Second, some UGGs have a label sewn into the interior of the left boot. This reflective label also includes a sun logo that changes color from black to white. Third, if the model you purchase does not have a sewn-in label, check the outsole of the left shoe; the brand's color-changing, reflective sticker can be placed there, instead [source: UGG Australia].