To distinguish the trapezius fibers from the splenius capitis fibers, you could ask your partner to perform what movement of the head and neck?

You could ask your partner to rotate his head slightly to the side you are palpating.

Trapezius it's a broad suface muscle that allows movement of the scapulae and movement of the spine.
splenius capitis is a deep muscle (in the posterior region of the neck) except  in the lateral side of the neck in which it gets between trapezius and sternocleidomastoid
.It pulls on the base of the skull to perform head extension and can help with lateral flexion and rotation of the cervical spine.

The trapezius muscle is a large muscle that is paired and extends longitudinally from the occipital bone down to the lower thoracic vertebrae and laterally to the scapular spine. On the other hand, the splenius capitis is a broad straplike muscle in the back of the neck. It pulls on the skull base from the cervical vertebrae and the upper thorax.

Based on their locations, there is an overlap between the two muscles. To be able to distinguish the trapezius muscle fiber from the splenius capitis fiber, ask your partner to rotate the head slightly to the side you are palpating.