यत्तु कामेप्सुना कर्म साहङ्कारेण वा पुनः। क्रियते बहुलायासं तद्राजसमुदाहृतम् ॥

yat tu kāmepsunā karma sāhankāreṇa vā punaḥ kriyate bahulāyāsaṁ tad rājasam udāhṛitam

But that action that is done by one longing for desires, or again with egoism, or with much effort, that is declared to be passionate.

In plain words: An action is driven by passion when it is done out of craving or to feed your ego. Doing it with great strain and struggle is also a sign of passion.