Look for the highlight on a sphere, like the ball of the nose, on corners, like the edge of the nose, and in corners like where the side of the nose meets the cheek . On the crest of a rounded form inside the center light. On the skin where the surface changes to wet (sweat) or oily. Usually a full value lighter than the form on which it sits.
Paint the highlight in as a teardrop that flows with the action. Shaped like a comet.
It can have soft edges on skin, or hard edges on metal.
Shifts toward the hue of the light source. The highlight is a reflection of the light source (the skin is not a perfect reflector).
It is an interest-getter.