The Fruit of Genuine Faith

How did the early church know that they could trust Saul of Tarsus (The Apostle Paul)? He was a violent persecutor of the Church at one point? How do you know that you can trust a person's profession of faith? What part does the Church play in helping to know these things? The local Church is essential to one's faith being healthy and and a true faith is essential for a thriving church!