Patanjali Yoga Sutra — प्रत्ययस्य परचित्तज्ञानम् ॥3.19॥

pratyayasya paracittajñānam॥

Through the knowledge of ultimate truth, the yogi gets mystical power to read the minds of others.

The individual mind, being the product of the universal mind, always exists and functions within the unlimited bounds of universality. For a yogi who is operating at advaitabhāva (a state of non-duality — ब्रह्मैवाहमस्मि), what could be a barrier? He is one with the Universal Consciousness. His sādhana is such that, he gains control over prakṛti, besides realizing the Puruṣa, the ultimate Truth. Hence, the minds of others are just totally transparent for a yogi owing to his knowledge of the universal mind.