How can you tell?
He is not retarded!
Good point.
Unfortunately, the “marry in” concept caught on throughout the Middle East and North Africa, regardless of religion. I recently read a book, “Jerusalem in the Time of Jesus,” by Joachim Jeremias, a German Biblical scholar. He mentioned that it was desirable for a priest to marry his niece, his brother’s daughter, because that way he could be sure of her pure bloodline.