Final answer:
The S1 dermatome is tested on the medial surface of the forearm and fingers, representing the area innervated by the eighth cervical nerve.
Step-by-step explanation:
The S1 dermatome is tested on the skin located at the medial surface of the forearm and extending out to the fingers. The dermatome represents the area of the skin innervated by the eighth cervical nerve. To test the S1 dermatome, a light touch stimulus, such as the soft end of a cotton-tipped applicator, is applied to the fingers and medial forearm, and the patient is asked to perceive the touch and report any differences in sensations.