Bhagavad Gita Chapter 14, Verse 13

अप्रकाशोऽप्रवृत्तिश्च प्रमादो मोह एव च | तमस्येतानि जायन्ते विवृद्धे कुरुनन्दन ||

aprakāśo 'pravṛttiś ca pramādo moha eva ca tamasy etāni jāyante vivṛddhe kuru-nandana

O descendant of the Kurus, when tamas increases, darkness, inactivity, negligence, and delusion arise.

Krishna describes the signs of increased tamas: 'Aprakāśo 'pravṛttiś ca pramādo moha eva ca'—darkness (aprakāśaḥ), inactivity (apravṛttiḥ), negligence (pramādaḥ), and delusion (mohaḥ). 'Tamasy etāni jāyante vivṛddhe'—these arise (jāyante) when tamas increases. This is how you recognize tamas: when you're in darkness, unable to see clearly, inactive, negligent, and deluded. When tamas is dominant, there's no illumination, no activity, no care, and no clarity. Understanding this helps you recognize when tamas is active and how it binds you through covering knowledge and attachment to negligence.