There are some bad fakes out there. The differences are easy to spot. Odd colourways, a Jordan sole etc etc. I'm not going into these variants, because i believe in the common sense of the public. Unfortunately there are also some really good fakes on the market. This guide will tell you what to look out for.
black: The space between the stitching and the rubber is very small in contrast with the real Yeezy shoes.
red: The rings for the laces should not be exactly the same colour as the leather. There should be a contrast between them. They should be a lighter colour than the leather.
purple: Looking at the sole might be the easiest way to spot a fake shoe. The fake shoes only have one row of "blocks" in the sole instead of two rows. This is also visible form the side. You won't see any inclusions on a fake shoe.
pink: A real Yeezy sole should have nice curved edges on the profiling on the soles. Fake ones have square edges. Also notice that the lines do not connect on the fake sole.
yellow: This is not a 100% proof way to tell that a shoe is fake, so you should not base your perception solely on this feature of the shoe. The little tag on the back of the shoe should be standing upright and not flat like in the picture of the fake shoe.
There are many more differences between fakes and real Yeezy shoes. This list is just to help you easily distinguish good fakes from real ones.