Here are five (5) major things to look for when buying a designer knock-off and tips for spotting a spoof.

1. STITCHING
Stitching can be a dead giveaway. Inconsistent stitching and sloppy construction in less visible areas, such as the underside of the bag or inside pockets, are details to look for immediately. Authentic designer bags tack on an extra zero on that price tag due to extra, carefully-crafted construction. They cost more because they are built more complexly. Counterfeit products usually do not have the care of construction that the real item has. If the construction looks simplistic, it's likely to be a phony.

2. MATERIALS
Many fakes are no longer made of PVC but of reasonably good leather, even lambskin; however, they are seldom as good as the materials labels like Gucci, Louis Vuitton, and Coach would use. Fake purses tend to be stiff and often have some sort of discoloration. When dealing with replica leather handbags, make sure the leather does not have an uneven look that is consistent. Also, people generally assume designer bags should be lined, which is why many replicas are lined. But, on the contrary, real purses are not. 

3. THE LABEL
Look for any kind of misspelling - at all -  on labels and tags. In any quality product, the label is stitched in, but in Gucci counterfeits (for example), a cheap hand tag will hang from the handle. Look at authentic purses online, and study the real tags. That way, it will be easier to spot the fake labels. Fake labels are generally cheaply made, unclear in impression, and has a lesser quality of stitching (and the attachment of the label). Two examples: for one, sometimes cheaply replicated Coach bags will include tags with misspelled and run-on words; secondly, Burberry bags will often be stamped "Burberrys" instead of "Burberry - London" in gold or silver on one side.

4. PACKAGING
Typically online bags or used bags do not have the original packaging, so this isn't much help.  However, if an online seller is advertising new packaging, be sure to check it out.  Luxury retailers put a great deal of care into packaging their products. This includes the works: tissues, wrapping paper, shopping bags, these extra touches are usually part of a real transaction. Look for items that have their original packaging. It's important that the box or bag the item comes in is neither too big nor too small, and is of the same brand as the purse you are purchasing. Makers of luxury goods seldom cram things into boxes or flimsy dust bags. Most luxury dust bags are made of thicker, soft cotton and flannel. Smaller items, like wristlets or wallets, will usually come in boxes.

5. DETAILS
Everybody knows that details can make or break a dress; well, the same goes for purses. The intricity of threads, leathers, fabrics, metals and stamping make a luxury bag what it is and should be...expensive. Most luxury items don't just come with authenticity cards; they also come with a product booklet with details on the details. Key things to look for when it comes to details: make sure the handles are of equal length, the logo is straight and firmly fastened, the details of the bag match up, and that the zippers are in full working order.

This can apply to all handbags, but specifically: Coach, DKNY, Kenneth Cole, Tommy Hilfiger, Ralph Lauren, Fendi, Gucci, Prada, and Armani.
 

Sources:
http://www.associatedcontent.com