First thing to look out for is the marks on the boost – the three-striped circles on the fakes and multi-striped on the retail pairs. On reps, they are arranged so as to form a cross and 7 of them are fully visible, while on the authentic pairs, 9 on them are fully-visible at the base and then 3 more are going vertically upward. Also, some more are partly visible on the sides. Not all pairs with 7 boost marks are fake but this is how most replicas have it.
Now, the texture of the boost varies on legit pairs and another comparison will be pictured below (with a different type of authentic boost). The difference you should be looking for is the shape of the pellets: on retail, pellets have more of a rounded shape, they’re more “bubbly” (as opposed to the flat boost on replicas) and less defined.
In order to understand, you’ll need to look at the shadows these pellets create and notice how, shape-wise, on fakes they’re closer to resemble a polygon, while authentic boost pellets have more of a rounded shape. Fake pellets are too “cut”.
Here’s another comparison to help you understand better
What you need to keep in mind is that there are several variations of retail boost, while fake boost is usually within the same lines. Here’s for instance another type of retail boost that’s not so commonly found but you can use your judgement and find the difference between the one pictured below and the fake one.