An allergy test now is going to cost way less then a lifetime of special suds, vet percribed food, treating systemic yeast infections and dealing with large smellt poo.
When a dog is on a high quality, minimal ingrediant food the dog will process it much better then a food full of fillers. Food like Old Roy, Alpo, ext are full of things that dogs cant digest, which is how we get bright yellow or red poo.
You feed less on a high quality food as well. A good option is Natures Domain from Costco, or 4health, taste of the wild, EVO, GoNatural, Orijen.