God as our life is versus our self.
Furthermore, the new man is versus our old man.
When we walk and live by the divine life in our new man, we have the sense of peace, feeling natural, restful, comfortable, and at ease.
When we have the inner sense of peace with the feeling of comfort, harmony, rest, joy, and liberty, we know that we are living in the spirit.
Romans 8:6 says, "The mind set on the flesh is death." The flesh includes our old man. Death is the issue of our flesh, including our old man.
Romans 8:6 shows us that death is the opposite of both life and peace. The mind set on the flesh is death, but the mind set on the spirit is life and peace.
Death gives us the senses of oldness, dryness, weakness, emptiness, depression, desolation, darkness, dullness, uneasiness, insecurity, discomfort, unnaturalness, sadness, conflict, etc.
When we have such negative senses, we are sensing death. By such negative senses, we know that we are not in the spirit.