How do we know the truth of the Gospel is genuine in someone's life?

The authenticity of one's faith in the Gospel is demonstrated through their lifestyle, as emphasized in 3 John 1:3, where John rejoices at hearing that Gaius walks in the truth. This walking in truth is not merely head knowledge but a heart transformation that leads to a life characterized by love, faithfulness, and good works. In essence, as James pointed out, faith without works is dead, indicating that true belief will always result in evidence of divine grace manifesting itself in one's actions and demeanor.