By their fruits?

"The one and only test of a valid religious idea, doctrinal statement, spiritual experience, or devotional practice was that it must lead directly to practical compassion. If your understanding of the divine made you kinder, more empathetic, and impelled you to express this sympathy in concrete acts of loving-kindness, this was good theology. But if your notion of God made you unkind, belligerent, cruel, or self-righteous, or if it led you to kill in God's name, it was bad theology."

Karen Armstrong, from "The Spiral Staircase : My Climb Out of Darkness"