How Do You Determine What A St. Bernard Is Allergic To?

You are actually doing the food trial in reverse.  The best thing to do is get your dog on a hypoallergenic diet available at your veterinarian (the pet store foods will not cut it).  There are 2 types of hypoallergenic diets--novel protein diets (where the protein and carbohydrate source are ones that dogs should not have encountered in previous diets---like duck and green pea), or hydrolyzed protein diets (the protein has been broken down to components that do not trigger allergies).  Then to find out exactly what he is allergic to start adding in one food source (like corn) at a time.  
Before starting the food trial work with your veterinarian to treat allergies symptomatically (redness and itching) and to clear secondary infections (bacteria, yeast, or mites).  During the food trial ONLY the diet food can be given.  It's like people with a peanut allergy--even 1 nut can cause a severe reaction.  It does take a long time--you need to do a food trial for a minimum of 8 weeks to assess response then can start adding food back in.
Dogs often have environmental allergies along with food allergies so if you do not see complete resolution with the food trial this is likely.
Allergies are about control not cure.  To successfully manage allergies you need to work with a veterinarian.

I think that you need to talk with the vet