How do i know if my cat has food allergies?

My cat has a very bad and neurotic habbit of scratching spots on his body until they become raw and bleeding. Usually its behind his neck, or in front of his ear. His is a indoor healthy kitty (no fleas!). My vet thought it was ringworm at first, but our cat tested negative. My vet mentioned that sometimes its an indication of food allergies. Does anyone know much about food allergies in cats? He is being fed perscription diet cat food r/d to help him maintain a healthy weight. Any info, including links, would be greatly appreciated! Thanks!