Here are some real vs. fake comparison photos of a Dsquared bag. Of course, this is just one example but the details below hold true for all styles. As this blog grows I will post more examples of different styles.
Example (click to enlarge):
First, you can dismiss the color difference in the handle. This bag was produced in two versions, one with the red handle and one with the tan handle. Below, is a photo of an authentic red handle bag on the runway. It is very hard to tell the real from the fake in the first comparison photo, so, as usual, the key lyes in the details. What is apparent from this first photo is the difference in quality of the denim. In the photos below you can also see the difference in quality of the leather trim.
The stitching is the give away in this example. Knockoffs often have crooked stitching that does not run parallel to the edge of the material. You will also notice that the stripes of the fake are skinnier and there is no "Genuine Leather" or "Made in Itlay" on this fake example.
Example (click to enlarge):
Now here we're getting into minute details, but, the studs on the bottom of the authentic bag are more rounded than those on the fake bag.
Also note the shape of the leather around the inner zipper and the length of the inner zipper.