How to know that one is not the body?

The following is an excerpt from a samvaad (dialogue) session with Acharya Prashant.

देहाभिमानपाशेन चिरं बद्धोऽसि पुत्रक ।
बोधोऽहं ज्ञानखंगेन तन्निष्कृत्य सुखी भव ॥

dehābhimānapāśena ciraṃ baddho’si putraka
bodho’haṃ jñānakhaḍgena tanniṣkṛtya sukhī bhava