How do I figure out if my dog has food allergies?

My pup has gastritis caused by a stomach ulcer ,he 's been hospitalized for 5 days but is still looking sad which can only mean he;s not feeling to well even with mediacation. Ive been thinking that maybe since yorkies have sensitive stomachs that maybe he is just allergic to something in either of his old foods. Right now were giving him canned Prescription Diet food ,which is basically water, pork liver and chicken liver.And I used to feed him Royal Canin for Yorkshire terriers which is basically chicken meal ,brown rice and rice.But I seriously have no idea , please hep!!