Reel in reference material.
A hammerhead shark has a very different head shape from that of a tiger shark or a great white. Studying detailed examples of your preferred species will help you to see its unique characteristics. To successfully capture distinct shapes, intricate skin details and the exact shape of those razor sharp teeth, you have to examine them closely.
With the exception of a local aquarium visit, observing these fish in person may not be a possibility. Thankfully, an endless supply of reference materials exists, like books, photos, models or even toys. You can explore thousands of reference images with resources like Adobe Stock.