The widest point is the cheekbones — clearly wider than both the forehead and the jaw. That is the defining feature and the thing to check first.
Then separate it from heart. A heart-shaped face has a wide forehead; a diamond has a narrow one. If both your forehead and your jaw are narrower than your cheekbones, it is a diamond.
The other frequent mix-up is oval. An oval face also tapers gently, but its widest point is much less pronounced and its forehead is a little wider than its jaw rather than similar to it. If the cheekbones are the obvious standout feature, it is a diamond.