Well, it'll only help if she has food allergies, plenty of dogs just have environmental allergies. Has she gone to the vet? Changing foods might be worth a shot, though, what is she currently being fed? And what are her symptoms? Has she always had allergies?
One method that's almost fool-proof is to feed a raw diet for a while since you can limit the diet to just one food at a time, know exactly where ingredients come from, etc. If you're not able to do that, try something similar (like the premade freeze-dried "raw" diets, 100% meat canned foods, etc.) or find a diet that doesn't share ingredients with her current food.
Either way, I can definitely sympathize, one of my dogs has food allergies (and another in the house, though not my dog, has environmental allergies...probably grass and/or pollen).