How to identify the fool?

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

Acharya Prashant (AP): Shri Krishna tells Arjun very early on in his epic discourse — “All that is ever happening is due to the three Gunas (attributes) of Prakriti (material nature). Yet, the foolish one thinks that he is the doer.”