The fruit of the Spirit of one who is filled with the Spirit of God:

Love, Joy, Peace, Patience, Kindness, Goodness, Gentleness, Faithfulness and Self Control.

The fruit of the narcissist:

Arrogance, Entitlement, Envy, Exploitiveness, Lack of Awareness, Blame Shifting, Grandiosity, Self Importance, Addiction to Praise, Honor and Admiration.

These are the ways we can know if someone is born-again or not. A child of God will have all the fruit of the Spirit of God as a lifestyle.

The narcissist will display all the traits of self-importance as a lifestyle.

The born-again believer may slip but is ashamed and confesses to be cleansed. 1 John 1:9

The narcissist never admits their flaws and punishes anyone who will point them out to help them repent.

Proverbs 28:13-14 "13 He who conceals his sins will not prosper, but whoever confesses and renounces them will find mercy. 14 Blessed is the man who is always reverent, but he who hardens his heart falls into trouble."