Whatever mental state is present, notice its signature. How do you feel it? Where in your body does it register? Is it in your face? Your head? Your chest?
Here, Sam Harris uses a somatic or embodied approach to deepen the experience of mind looking at mind. It is a modern insight, that every metnal state has a somatic representation ,or signature. All emotions are "embodied" (eg Raja 2022).
This way, for the meditator the internal state is made more specific and concrete, rather than staying a thought construct.
This instruction is also a search operation. It is an act of searching for manifestations of "Self". It is analogous to the search instruction to look for the Self in the body, eg in as used by Daniel P Brown:
Recall your sense of Self. Then, using your high-speed awareness, try to find this Self anywhere in your body.
Raja, S. (2022). The Practice of Embodying Emotions: A Guide for Improving Cognitive, Emotional, and Behavioral Outcomes. North Atlantic Books.