Sounds like an open door to introduce them to Elder Porphyrios, or St. Paisios. Compare track records, wisdom, and "insight". If the prophet is no a 100%er (who should be stoned by strictly literal Biblical standards), if he does not have the active and manifest grace that attended to the lives of our more recent Saints and elders…then maybe having examined the lives of a few he might realize he's been swimming in the wrong baptistry…and want to investigate the faith which can to this day produces holy men and women of New Testament stature.
Of course such counting coup "contests"/challenges might be crass/out of place in a given context….but hey, you never know if The Young Man, the Guru, and Elder Paisios, Wounded by Love, or The Aescitic of Love might make an impression. Then there's the whole Holy fire in Jerusalem, the annual cloud on Mt. Tabor, the Jordan River flowing backwards at Theophany, etc.