Bhagavad Gita Chapter 14, Verse 11

सर्वद्वारेषु देहेऽस्मिन्प्रकाश उपजायते | ज्ञानं यदा तदा विद्याद्विवृद्धं सत्त्वमित्युत ||

sarva-dvāreṣu dehe 'smin prakāśa upajāyate jñānaṁ yadā tadā vidyād vivṛddhaṁ sattvam ity uta

When illumination of knowledge shines through all the gates of the body, then it should be understood that sattva has increased.

Krishna describes the sign of increased sattva: 'Sarva-dvāreṣu dehe 'smin prakāśa upajāyate'—when illumination (prakāśaḥ) arises (upajāyate) through all the gates (dvāreṣu) of the body. 'Jñānaṁ yadā tadā vidyād vivṛddhaṁ sattvam ity uta'—when knowledge (jñānam) shines, then one should know (vidyāt) that sattva has increased (vivṛddham). This is how you recognize sattva: when clarity and knowledge shine through all your senses—your eyes see clearly, your mind understands, your actions are purposeful and harmonious. When sattva is dominant, there's illumination in everything you perceive and do. Understanding this helps you recognize when sattva is active and how to cultivate it more.