By direct perception of the content of the mind, the ability to understand others’ minds.
The common denominator is that we are human beings and human beings operate through their minds. First we must shed light on our own mind, developing awareness of its fluctuating contents as a detached observer which is often termed the Witness Consciousness. Only once we are aware of and know our own mind are we able to also know how the mind of others work.
First we must clearly see our own mind and put it aside in order to focus and pick up on the subtleties of another mind.
As the great Guru, Yogarishi Dr Swami Gitananda Giri used to respond to people asking whether he could read minds: “I am not interested in reading dirty books”.
All Siddhis when they spontaneously arise in our consciousness should only be used for creating greater empathy within us. The aim of Yoga (in order to experience an existential and transcendental union with All that is) is to drop the mind.