The stitching on fake BAPE shark hoodies is usually much less dense and rich. The spacing between the letters is suspect. The "WGM" patch is also a giveaway. Look for the logo on the side of the hoodie - it should say "World Gone Mad" in Latin. This is one of the most popular BAPE items. In fact, the logo is so iconic that it is used on many Bape products.
There are three major signs of a fake Bape shark hoodie. The first one is the stitching. The stitching is not as dense as that of a real Bape. The stitching around the neckline of a fake Bape shark hoodie is also suspect. The stitching also lacks a certain pattern, which is unique to a genuine Bape. Lastly, the stitching on the top is poor.