When the light of knowing arises in all the gates of this body, then one should understand that sattva has grown strong.
Explanation
Krishna is describing to Arjuna how to recognize each of the three qualities of nature (guṇa) in oneself. Here the sign of sattva (clarity, goodness) is illumination and knowing that shows up through all the "gates" of the body, which commonly means the senses and the mind's faculties of perception. Commentators differ on whether this knowing is ordinary clear awareness or a step toward spiritual insight, but all agree it marks sattva's strength.