To know that you are neither body nor mind, watch yourself steadily and live unaffected by your body and mind, completely aloof, as if you were dead. It means you have no vested interests, either in the body or in the mind.
Just remain unaffected. This complete aloofness, unconcern with mind and body is the best proof that at the core of your being you are neither mind nor body.
What happens to the body and the mind may not be within your power to change, but you can always, put an end to your imagining yourself to be body and mind. Whatever happens, remind yourself that only your body and mind are affected, not yourself.