Yogi know everything that is in that mind, past, present, and future.

Verse 3.19

प्रत्ययस्य परचित्तज्ञानम् ॥१९॥

pratyayasya para-citta-jñānam ॥19॥

He acquires the ability to understand the minds of others.

Verse 3.20

न च तत् सालम्बनं तस्याविषयी भूतत्वात् ॥२०॥

na ca tat sālambanaṁ tasya-aviṣayī bhūtatvāt ॥20॥

A yogi who is able to read the minds of others in general, can also, if necessary, precisely identify specific contents which are beyond the reach of the mind.

Through his purity of consciousness, the yogi realizes directly the nature of his own mind and consciousness, and also that of others.

The word sakshatkaranat used in 3.18, means seeing reality. 

The word pratyaya means perceiving the content of the mind. Both convey the same meaning. By mastery over his own mind and consciousness, the yogi develops clairvoyance and can read the minds of others.

He would not know the contents of the mind by making a Samyama on the body. There would be required a twofold Samyama, first on the signs in the body, and then on the mind itself. The Yogi would then know everything that is in that mind, past, present, and future.

PATANJALI YOGA SUTRAS