Om Swami
How do I know what is a sin and what is my dharma?
Know your duty to know your dharma
Understanding your duty, whether moral, social or spiritual, is how you know your dharma. Actions done with detachment, not putting yourself first and for the world, another person or the greater good, do not tie you down.
When you understand your duty, your moral duty, your social duty and your spiritual duty, then you know your Dharma.