Good luck!
References :
Personal Experience

1. Price – If it’s really inexpensive, it’s probably fake. There may be some little old lady who died and left designer bags to her children who don’t know what they’ve got, but that’s rarer now with more brand/value awareness.

2. Design – Designer products come in a limited combination of colors and patterns. Learn the patterns and designs that you like, go to the designer store (if there’s one near you) and look at the real thing. That will give you an "eye" for the fakes. Patterns will be too big, too bold, or not big and bold enough. Color schemes will be different.

3. Description – If the description says: "like <brand name>" or "compares to <brand name>", or similar language, it’s a knock-off. In some industries, fakes are okay. In designer and brand name products, ALL fakes are illegal, even if they’re actually described as fakes.

4. Quality – As others have mentioned, how the product is put together can identify a fake. But many people don’t know the fine details that differentiate designer-quality goods. For clothing it may be additional stitching in the seams, hand-sewing vs. machine-sewing, the quality of the materials, etc. For handbags, it may be the type of hardware and how much hardware is on the bag, quality of stitching, quality of leather, size, shape, color, pattern, etc. The inside liner may have to be a specific color. Logos alone do not always help you identify the fakes.

5. Labels – Read labels CAREFULLY. Sometimes the designer’s name is misspelled. The grammar isn’t correct. Trademark information, if appropriate, is missing. The logos or font face are wrong.

6. Source – Are you buying this item from a reputable source? Be aware that length of time in business, price for items, and number of satisfied customers does not make a source reliable. Lots of buyers who don’t know how to identify a fake will accept a fake as real and be very happy with the product because they think it’s genuine.

If you’re buying on Ebay, for example, you can go to various discussion groups for the type of items you’re interested in. There are always discussion threads on fakes, identifying fakes, and how to avoid rip-offs. Sellers or knowledgeable buyers who know how to spot specific fakes may have information on their "About Me" pages.

If you’re interested in identifying a specific item at auction as genuine or fake, you can always join the discussion group and ask.

It does take an expert to identify others.

7. Sometimes you can’t identify a genuine brand name item without expert knowledge or resources. In particular, fine jewelry may be marked "Tiffany" or "Cartier" and be fake. Unless you know the specific year/mark identifiers or the models of jewelry produced by the company, it’s not always possible to identify a fake. Tiffany & Co. charge $500 to authenticate items (last time I checked a couple of years ago). Rolex watches are notorious for fakes. I’ve heard that Rolex has a list of x ways to identify a genuine Rolex, some of which require taking the back off the watch and inspecting the works. Only an expert should attempt this type of verification or you risk damaging the watch. Fine jewelry stores that routinely sell Rolex should have this list and some pawn shops that pawn or buy high-end watches may have this list. It’s very unlikely that they’ll show or give you a copy of the list because if the list got into the wrong hands, it would be a way for a faker to know what to fake.

The bottom line is — if you want to know for certain that you’re buying the genuine article, your best bet is to 1) pay full price at the designer’s store or authorized source, or 2) really dig into the details of the item you desire so you know it inside and out.
References :